THE SENSES OF ANIMALS.
It is part of the essential nature of an animal to be receptive and
responsive. The forces of nature rain their influence upon it; and it
reacts to their influence in certain special ways. Other organisms
surround it, compete with it, contend with it, strive to prey upon it,
and occasionally lend it their aid. It has to adjust itself to this
complex environment.
There are two kinds of organic response--one more or less permanent, the
other temporary and transient. We have already seen something of the
former, by which the tissues (the epidermis of the oarsman's hand, and
the muscles of his arm) respond to the call made upon them. The response
is here gradual, and the effects on the organism more or less enduring.
This, however, is not the kind of response with which we have now to
deal. What we have now to consider is that rapid response, transient,
but of the utmost importance, by means of which the organism directly
answers to certain changes in the environment by the performance of
certain activities. The parts specially set aside and adapted to receive
special modes of influence of the environment are the sense-organs. We
human folk get so much pleasure from and through the employment of our
sense-organs, that it is important to remember that the primary object
of the process of reception of the influences from without was not the
æsthetic one of ministering to the enjoyment of life by the recipient
organism, but the essentially practical one of enabling that organism to
respond to these influences. In other words, the _raison d'être_ of the
sense-organs is to set agoing suitable activities--activities in due
response to the special stimuli.
In this chapter we shall consider the modes in which the special
sense-organs are fitted to receive the influences of the environment,
deferring to a future chapter the consideration of the resulting
activities. For the present we take these activities for granted,
observing them only in so far as they give us a clue to the
sense-reaction by which they are originated. In this chapter, too, we
shall deal, for the most part, with the physiological aspects of
sensation. In all other organisms than ourselves, that is to say, than
each one of us individually for himself, the psychological
accompaniments of the physiological reactions of the sense-organs are
matters of inference. Still, so closely and intimately associated are
the physiological and the psychological aspects, that the exclusion of
all reference to the latter would be impracticable, or, if practicable,
unadvisable. What is practicable and advisable is to remember that, even
if the two are mentioned in a breath, the physiological and the
psychological belong to distinct orders of being.
In addition to the time-honoured "five senses," there are certain
_organic sensations_, so called, which take their origin within the
body. These are, for the most part, somewhat vague and indefinite. They
do not arise immediately and in direct response to changes in the
environment, but indicate conditions of the internal organs. Such are
hunger, thirst, nausea, fatigue, and various forms of discomfort.
Although they are of vital importance to the organism, prompting it to
perform certain actions or to desist from others, they need not detain
us here.
More definite than these, but still of internal origin, is the _muscular
sense_. This, too, is of continual service to every active animal. By it
information is given as to the energy of contraction of the muscles, and
of the amount of movement effected--not to mention the rapidity and
duration of the muscular effort. By it the position, or changes of
position, of the motor-organs are indicated. It is obvious, therefore,
that the sensations obtained in this way, some of which are exceedingly
delicate, are an important guide to the organism in the putting forth of
its activities. It is through the muscular sense that we maintain an
upright position. It is through an educated and refined muscular sense
that the juggler and the acrobat can perform their often surprising
feats. Concerning the physiology of the muscular sense, we have at
present no very definite knowledge. Some have held that we judge of
muscular movements by the amount of effort required to initiate them;
but it is much more probable that there are special sensory nerves,
whose terminations are either in the muscles themselves or in the
membranes which surround them.
* * * * *
We come now to the special senses. Of these we will take first the
_sense of touch_. Through this sense we are made aware of bodies solid
or liquid (or perhaps gaseous) which are actually in contact with the
skin or its infoldings at the mouth, nostrils, etc. There are
considerable differences in the sensitiveness of the skin in different
parts of its surface; some parts, like the filmy membrane which covers
the eye, being very sensitive, while others, like the horny skin that
covers the heel of a man who is accustomed to much walking, are
relatively callous. Different from this is the delicacy of the sense of
touch. This delicacy is really the power of discrimination, and
therefore involves some mental activity. But it is also dependent upon
the distribution of the recipient end-organs of the nerve. The highest
pitch of delicacy is reached in the tip of the tongue, which is about
sixty times as delicate as the skin of the back. The power of
discrimination is tested in the following way: The points of a pair of
compasses are blunted, and with them the skin is lightly touched. When
the points are close together, the sensation is of one object; when they
are more divergent, each point is felt as distinct from the other. On
the thigh and in the middle of the back, two distinct points of contact
are not felt unless the compass-tips are about 2-1/3 inches (67.7
millimetres) apart. When the divergence is 2 inches, they are felt as
one. With the tip of the tongue, however, we can distinguish the two
separate points when they are only 1/25 of an inch (1.1 millimetre)
apart. For the finger-tip the distance is about 1/12 of an inch (2
millimetres); for the tip of the nose, about 1/4 of an inch (6.8
millimetres); for the forehead, a little less than an inch (22.6
millimetres); and so on. Shut your eyes, and allow a friend to draw the
compass with the points about 1/2 an inch apart, from the forehead to
the tip of your nose, or (setting the points about 1/4 of an inch apart)
from the ball of your thumb to the finger-tip. The increasing delicacy
and power of discrimination is readily felt, and it is difficult to
believe that the compasses are not being slowly opened.
It is beyond the purpose of this chapter to describe minutely the nature
and structure of the nerve-ends in the sense-organs. This is a matter of
minute anatomy, or histology. A full description of them as they occur
in man will be found in any standard text-book of physiology; while Sir
John Lubbock's "Senses of Animals" gives much information concerning,
and many illustrations of, the minute structure of the sense-organs in
the invertebrates. Here I can only touch very briefly on some of the
more important points.
One of the larger nerves of the body (e.g. the sciatic nerve), consists
of a bundle of nerve-threads collected from a considerable area; some of
these (motor threads) end in muscles, others (sensory threads) in the
skin or its neighbourhood. Each nerve-thread has a central axis-fibre,
which is surrounded by a fatty, insulating medullary sheath, and this by
a delicate primitive sheath. In some parts of the skin the sensory
nerve-threads lose their medullary sheath, and end in very fine branches
between the cells of the tissue. In other cases the cells near their
termination are specially modified to form tactile cells, or tactile
corpuscles, in contact with or surrounding the axis-fibre or its
expansion (Fig. 23).
[Illustration: Fig. 23.--Tactile corpuscles.
1. In the beak of a goose. 2. In the finger of a man. 3. In the
mesentery of a cat.]
Hairs are delicate organs of touch, though, of course, this is not their
only function. They act as little levers embedded in the skin.
Turning now to the vertebrate animals other than man, we find in them a
sense of touch closely analogous to our own. As in us, so in them, the
specially mobile parts are eminently sensitive and delicate; for
instance, the lips in many animals, such as the horse, and the
finger-like organ at the end of the elephant's trunk. In some of them
special hairs are largely developed as organs of touch, as in the
whiskers of the cat and the long hairs on the rabbit's lip. With the aid
of these the rabbit finds its way in the darkness of its burrow; and it
is said that, deprived of these organs, the poor animal blunders about,
and is unable to steer its course in the dark.
The wing of the bat is very sensitive to touch; and it is supposed that
it is through this sense that the bat is able to direct its course in
the darkness of caves. Miss Caroline Bolton thus describes an
experimental trial of this power of the bat at which she was herself
present. A room, about twenty feet by sixteen, was arranged with strings
crossing each other in all directions so as to form a network with about
sixteen inches space between the strands. To each string was attached a
bell in such a way that the slightest touch would make it ring. One
corner of the room was left free for those who were present at the
experiment. A bat, measuring about one foot from the tip of one wing to
that of the other, was let loose in the room when it was quite dark,
"and it was distinctly heard flying about all over the room, but never
once did it touch a string or stop flying. It several times came quite
near to the spectators, so that they could feel the vibration of the air
in their faces. The experiment was continued for half an hour. Then,
when the door was opened and light let in, the bat stopped flying, and
settled down in the darkest corner." Now, here it may be said that,
although the room was dark to human spectators, there may have been
light enough for a bat to see his way. The cruel experiments of
Spalanzani, however, who put out the eyes of bats and obtained a similar
result, seem to show that the animal is guided by some sense other than
that of sight.
[Illustration: Fig. 24.--Touch-Hair of insect.
t.h., touch-hair; cu., cuticle; h.y., hypodermis; g., ganglion-cell
connected with nerve passing into the cavity of the touch-hair (after
Miall). The ganglion is often surrounded by several--eight or
less--accessory cells, which are not figured here.]
The crustaceans and many insects are covered with a dense armour, and it
might be supposed that in them there could be no sense of touch. But
this sense is by no means absent. Seated on the tough integument are
delicate little hairs, to the base of which a nerve-fibril passes
through a perforation in the integument. These are specially numerous in
the antennæ of insects.
In yet lower organisms we know in some cases the manner in which they
are sensitive to touch; but in a great number of cases, although
observation shows that they are thus sensitive, we know nothing definite
as to how the surface is specially fitted to receive the stimuli. Even
the primitive am[oe]ba, however, is sensitive in the sense spoken of on
p. 8; that is to say, it reacts under the influence of a stimulus.
* * * * *
Closely associated with the sense of touch is the _temperature-sense_.
Goldschneider and others have shown that on the skin of the human hand,
for example, there are special points that are sensitive to heat and
cold. Some of these little specialized areas are sensitive to cold;
others are sensitive to heat; and neither of these seem to be sensitive
to pressure. It therefore seems probable that special nerve-fibrils are
set apart for the temperature-sense; but of the manner in which these
fibrils terminate little or nothing is known.
Let us note that this temperature-sense, unlike the sense of touch, may
make us aware of distant bodies. It is, then, what we may term a
_telæsthetic_ sense in contradistinction to a contact-sense. It is
stimulated by a molecular throb; the throbbing body may be in contact,
but it may be as distant as the sun, in which case the molecular
pulsations are brought to us on waves of æther. Whether these waves act
directly on the nerve end-organs, or indirectly on them through the
warming of the skin-surface in which they terminate, we cannot say for
certain. But if the hand be held before a heated stove and be sheltered
from the heat by a screen, the removal of the screen, even for the
fraction of a second, gives rise to a strong stimulation of the
temperature-sense, though the skin-surface be not appreciably raised in
temperature. Hence it is probable that the end-organs are stimulated
directly, and not indirectly.
Concerning the temperature-sense in the lower animals, nothing definite
is known. But it is impossible to see our familiar pets basking in the
sunshine, or a butterfly sunning itself on a bright summer's day,
without feeling confident that the temperature-sense is a channel of
keen enjoyment. As before mentioned, however, this is not to be regarded
as the primary end in sensation. The primary end is not life-enjoyment,
but life-preservation. And we must regard the temperature-sense as
developed in the first instance to enable the organism to escape from
the ill effects of deleterious heat or cold, and to seek those
temperature-conditions which are most helpful to the continued and
healthful fulfilment of the process of life.
* * * * *
The _sense of taste_ is called into play by certain soluble substances,
or liquids, which must come in contact with the specialized
nerve-endings. Under normal circumstances, the sense of taste is closely
associated with that of smell, the result of the combination of the two
special senses being a _flavour_. The _bouquet_ of a choice wine, the
flavour of a peach, involve both senses; quinine involves taste alone;
and garlic and vanilla are nearly, if not quite, tasteless,--what we
call their taste is in reality their action on the organ of smell.
It is difficult to classify tastes. Sweet, bitter, salt, alkaline, sour,
acid, astringent, acrid,--these are the prominent and characteristic
varieties.
[Illustration: Fig. 25.--Taste-buds of rabbit.
i., section across part of the pleated patch (enlarged); ii., taste-buds
further enlarged.]
This sense is generally localized in or near the mouth; in us mainly in
the tongue. One manner, but not the only manner, in which the nerves in
this region terminate is in the minute flask-shaped taste-buds, which
have near one end, where they reach the surface, a funnel-shaped
opening, the taste-pore. They are made up of elongated cells, some of
which near the centre are spindle-shaped, and are called taste-cells.
They are found chiefly round the large circumvallate papillæ; but in the
rabbit and some other animals they are collected in the folds of a
little ridged or pleated patch--the _papilla foliata_--on each side of
the tongue near the cheek-teeth.
It is probable that the stimulation of the end-organs of taste is
effected by the special mode of molecular vibration due to the chemical
nature of the sapid substance. Mr. J. B. Haycroft, in a paper read
before the Royal Society of Edinburgh,[EO] suggests that "a group of
salts of similar chemical properties have their molecules in a similar
vibrating condition, giving rise to similar colours and similar tastes."
"Thus the chlorides and sulphates of a series of similar
elements--called a group of elements by Mendeljeff--have similar
tastes."
The delicacy of the sense of taste in man has been the subject of
investigation by Messrs. E. H. S. Bailey and E. L. Nichols.[EP] They
give the following table:--
I. Quinine--
Male observers detected 1 part in 390,000 parts of water.
Female " " 1 " 456,000 " "
II. Cane-sugar--
Male observers " 1 " 199 " "
Female " " 1 " 204 " "
III. Sulphuric acid--
Male observers " 1 " 2,080 " "
Female " " 1 " 3,280 " "
IV. Bicarbonate of sodium--
Male observers " 1 " 98 " "
Female " " 1 " 126 " "
V. Common salt--
Male observers " 1 " 2,240 " "
Female " " 1 " 1,980 " "
The above figures represent means or averages of a great number of
individuals. There was very considerable variation for some tastes. In
the case of the bitter of quinine, the maximum delicacy was the
detection of 1 part in 5,120,000 parts of water; the minimum 1 part in
456,000 parts of water. Except in the case of salt, the sense was more
delicate in women than in men. It is not stated whether the men tested
were smokers.
It does not seem necessary to say anything concerning the sense of taste
in the lower mammalia.
In birds and reptiles the sense of taste does not appear to be highly
developed. Parrots are, perhaps, better off in this respect than the
majority of their class; and the ducks have special organs on the edges
of the beak, which seem to minister to this sense. A python at the
Zoological Gardens, partially blind owing to a change of skin, is said
to have struck at an animal, but to have only succeeded in capturing its
blanket. This, however, it constricted, and proceeded to swallow with
abundant satisfaction.
It may here be mentioned that the scales and skin of many fishes are
provided with sense-organs which very closely resemble the taste-buds of
higher animals. They occur in the head and along the "lateral line"
which runs down the side of the fish, and may be readily seen, for
example, in the cod. Mr. Bateson's[EQ] careful observations at Plymouth
gave, however, no indication of the possession of an olfactory or
gustatory function, and their place in the sensory economy of the fish
remains problematical. In or near the mouth similar end-organs are found
to be somewhat variously developed in different fishes--on the palate
and lips, on the gill-bars, more rarely on the tongue, and on the
barbels of the rockling and the pout. How far any or all of these have a
gustatory function remains to be proved.
Anglers and fishermen, however, from their everyday experience, and
naturalists from special observations, do not doubt that fishes have a
sense of taste. Professor Herdman's recent experiments on feeding fishes
with nudibranchs[ER] (naked molluscs) seem to show, for example, that
the fishes concerned, including shannies, flat-fish, cod, rockling, and
others, have a sense of taste leading them to reject these molluscs as
nasty. They show, too, that some of the nudibranchs (_Doris_, _Ancula_,
_Eolis_) are protected by warning coloration.
Our knowledge of the sense of taste among the lower (invertebrate)
animals is imperfect, and is largely based rather on observation of
their habits than on the evidence of anatomical structure. Here, again,
comes in the difficulty of distinguishing between taste and smell. But
even if the caterpillars which refuse to eat all but one or two special
herbs, or the races of bloodsuckers which seem to have individual and
special tastes, are guided in part by an olfactory sense, there is much
evidence which seems to admit of no alternative explanation. Moisten,
for example, the antennæ of a cockroach with a solution of Epsom salts
or quinine, and watch him suck it off; or repeat F. Will's experiments
on bees, tempting them with sugar, and then perfidiously substituting
pounded alum. The way in which these little insects splutter and spit
suggests that, whatever may be the psychological effect, the
physiological effect is analogous to that produced in us by an
exceedingly nasty taste. Here smell would seem to be excluded. Forel,
moreover, mixed strychnine with honey, and offered it to his ants. The
smell of the honey attracted them, but when they began to feed, the
effect of the taste was at once evident.
The organs of taste in insects are probably certain minute pits, in each
of which is a delicate taste-hair, which, in some cases, is perforated
at the free end. They occur in the maxillæ and tongue in ants and bees,
and on the proboscis of the fly.
In many of the invertebrates, the crayfish and the earthworm, for
example--to take two instances from very different groups--observation
seems to show that a sense of taste is developed, for they have marked
and decided food-preferences. Nevertheless, the existence of special
organs for this purpose has not been definitely proved.
The sense of taste no doubt ministers to the enjoyment of life. But,
presumably, it has been developed in subservience to the process of
nutrition. Primarily, taste was not an end in itself, but was to guide
the organism in its selection of food that could be assimilated. Nice
and nasty were at first, and still are to a large extent, synonymous
with good-for-eating and not-good-for-eating. With unwonted substances,
however, its testimony may be false. Sugar of lead is sweet, but fatal.
Brought to a new country, cattle often eat, apparently with relish,
poisonous plants. Still, under normal circumstances, the testimony of
taste is reliable.
* * * * *
The _sense of smell_ is, to a large extent, telæsthetic. It is true that
the stimulation of the end-organs is effected by actual contact with the
odoriferous vapour. But since this vapour may be given off from an
odoriferous body at some distance from the organism, such as a flower or
a decomposing carcase, it is clear that the sense gives information of
the existence of such bodies before they themselves come in contact with
us. Primitively, we may suppose that it was developed in connection with
that sense of taste with which, as we have seen, it is so closely
associated. In this respect smell is a kind of anticipatory taste. But
it has now other ends, apart from those which are purely æsthetic. In us
it may serve as a warning of a pestilential atmosphere; in many
organisms, such as the deer, it gives warning of the presence of
enemies; in many again, and some insects among the number, it is the
guiding sense in the search for mates.
The organ of smell in ourselves and in all the mammalia is the delicate
membrane that covers the turbinal bones in the nose. It contains cells
with a largish nucleus, around which the protoplasm is mainly collected.
A filament passes from this to the surface, and ends in a fine hair or
cilium (or a group of hairs or cilia in birds and amphibia); a second
filament runs downwards into the deeper parts of the tissue, and may
pass into a nerve-fibril.
In us and air-breathing creatures, the substance which excites the
sensation of smell must be either gaseous or in a very fine state of
division; but in water-breathers the substance exciting this
sensation--or, in any case, one of anticipatory taste--may be in
solution. The sensitiveness of the olfactory membrane is very
remarkable. A grain of musk will scent a room for years, and yet have
not sensibly lost in weight. Drs. Emil Fischer and Penzoldt found that
our olfactory nerves are capable of detecting the 1/4,600,000 part of a
milligramme of chlorophenol, and the 1/460,000,000 part of a
milligramme, or about one thirty-thousand-millionth of a grain, of
mercaptan. It may be that to such substances our olfactory sensibility
is especially delicate.
Not much is known concerning the manner in which the end-organs of smell
are stimulated. As in the case of taste, it is probably a matter of
molecular vibration; and Professor William Ramsay has suggested that the
end-organs are stimulated by vibrations of a lower order than those
which give rise to sensations of light and heat. He has also drawn
attention to the fact that to produce a sensation of smell, the
substance must have a molecular weight at least fifteen times that of
hydrogen.
It is well known that the sense of smell is in some of the mammalia
exceedingly acute. The dog can track his master through a crowded
thoroughfare. The interesting experiments of Mr. Romanes[ES] show that,
under ordinary conditions of civilized life, the smell of boot-leather
is a factor, and the dog tracks his master's boots. In one case, the
boots were soaked in oil of aniseed, but this to us powerful scent did
not overcome the normal odour of the master's boots. Mr. W. J. Russell,
in a subsequent number of the same periodical, describes how his pug
could find a small piece of biscuit by scent, and this odour of biscuit
was not overmastered by a strong smell of eau-de-Cologne. Deer-stalkers
know well how keen is the sense of smell in the antlered ruminants.
We must not, however, be too ready to conclude, from these observations,
that the olfactory membrane is absolutely more sensitive in such animals
than it is in man. It may well be that, though they are so keen to
detect certain scents, they are dull to those which affect us
powerfully. It is quite possible that the odour of aniseed or
eau-de-Cologne is--possibly from the fact that their end-organs are not
attuned to these special molecular vibrations--out of their range of
smell. Their special interests in life have led to the cultivation of
extreme sensibility to special tones of olfactory sensation. Under
unusual circumstances, man may cultivate unwonted modes of utilizing the
sense of smell. A boy, James Mitchell, who was born blind, deaf, and
dumb, and who was mainly dependent on the sense of smell for keeping up
some connection with the external world, observed the presence of a
stranger in the room, and formed his opinion of people from their
characteristic smell. On the whole, therefore, we may, perhaps, conclude
that the variations in sensitiveness are mainly relative to the needs of
life.
In birds the sense of smell is but little developed, notwithstanding all
that most interesting naturalist, Charles Waterton, wrote on the
subject. Vultures seem unable to discover the presence of food which is
hidden from their sight. Probably reptiles share with them this dulness
of the sense of smell.
It has already been remarked that, in the case of aquatic animals, there
is probably little distinction between taste and smell. It would be
well, perhaps, to restrict the word "smell" to the stimuli produced by
vapours or air-borne particles, and to use the phrase "telæsthetic
taste," or simply "taste," for those cases where the effects are
produced through the medium of solution. In this case, however, the
point to be specially noticed is that taste in aquatic animals becomes a
telæsthetic sense, informing the organism of the presence of more or
less distant food. Thus, if you stir with your finger the water in which
leeches are living, they will soon flock to the spot, showing that the
telæsthetic sense is associated with an appreciation of direction. If a
stick be used to stir the water, they do not take any notice of it. Mr.
W. Bateson[ET] has shown that there are many fishes, among which are the
dog-fish, skate, conger eel, rockling, loach, sole, and sterlet, which
habitually seek their food by scent (telæsthetic taste), aided to some
extent by touch, and but little, if at all, by sight. "None of these
fishes ever starts in quest of food when it is first put into the tank,
but waits for an interval, doubtless until the scent has been diffused
through the water. Having perceived the scent of food, they swim vaguely
about, and appear to seek it by examining the whole area pervaded by the
scent, having seemingly no sense of the direction whence it proceeds." I
venture to think that further observation and experiment may show that
such a sense of direction does in some cases exist. Some years ago I was
fishing in Simon's Bay, at the Cape, with a long casting-line. The sea
was unusually calm, and the water clear as crystal. Beneath me was a
clear patch of granite, two or three yards across, surrounded by tangled
seaweed. Evening was coming on, and I was just going to put up my tackle
when I saw a long dark fish slowly sail into the open space and take up
his position at one side. My line was out, baited, I think, with a piece
of cuttle-fish, and I tried to draw it into the clear space, but only
succeeded in bringing it to within a foot or so of the side furthest
from the fish. There it got hitched in the weed; but the fish being
still undisturbed, I awaited further developments. After two or three
minutes the fish slowly turned, crossed the pool, and remained
motionless for a few moments; then he proceeded straight to the bait;
and in a few minutes I had landed a dog-fish between four and five feet
long. I did not then know that the dog-fish sought its food mainly or
solely by scent (taste); but in any case I do not think in this instance
he could have seen the bait, hidden as it was amid the seaweed.
Although I am aware, and have already mentioned, that Mr. Bateson's
observations do not support the view that the sense-organs of the
lateral line minister to this telæsthetic sense, still I think that
further observations and experiments may show that these sense-organs
are "olfactory," and that the lateral development may be in relation to
the appreciation of the direction in which the food lies. It is,
however, a difficult matter to determine, and the few experiments I have
made are so far inconclusive.
Much has been written concerning the sense of smell in insects. That
they possess such a sense few will be disposed to doubt. The classical
observations of Huber show that bees are affected by the smell of honey,
and that the penetrating odour of fresh bee-poison will throw a whole
hive into a state of commotion. He was of opinion that the impunity with
which his assistant, Francis Burnens, performed his various operations
on bees was due to the gentleness of his motions, and the habit of
repressing his respiration, it being the odour transmitted by the breath
to which the bees objected. Sir John Lubbock formed a little bridge of
paper, and suspended over it a camel's-hair brush containing scent, and
then put an ant at one end. She ran forward, but stopped dead short when
she came to the scented brush. Dr. McCook introduced a pellet of
blotting-paper saturated with eau-de-Cologne into the neighbourhood of
some pavement-ants, who were engaged in a free fight. The effect was
instantaneous; in a very few seconds the warriors had unclasped
mandibles, relaxed their hold of their enemies' legs, antennæ, or
bodies.
The correct localization of the sense of smell has been a matter of
difficulty. Kirby and Spence localized it at the extremity of the
"nose," between it and the upper lip. That the nose, they naïvely
remark, corresponds with the so-named part in mammalia, both from its
situation and often from its form, must be evident to every one who
looks at an insect. Lehman, Cuvier, and others, misled by the fact that
the organ of smell is in us localized at the entrance of the air-track,
supposed that at or near the spiracles of insects were the organs of
smell. Modern research tends more and more clearly to localize the sense
of smell, as first suggested by Réaumur, in the feelers or antennæ, and
in some cases also in the palps. If the antennæ of a cockroach be
extirpated or coated with paraffin, he no longer rushes to food, and
takes little notice of, and will sometimes even walk over,
blotting-paper moistened with turpentine or benzoline, which a normal
insect cannot approach without agitation. There can be little doubt that
it is by means of its large branching antennæ that the male emperor moth
(_Saturnia carpini_) is able to find its mate.[EU] If a collector take a
virgin female into a locality frequented by these moths, he will soon be
surrounded by twenty or thirty males; but if the moth be not a virgin,
he will at most see one or two males. The sense of smell is thus
delicate enough to distinguish the fertilized from the unfertilized
female, and has associated with it a sense of direction by which the
insect is guided to the right spot. Carrion flies whose antennæ have
been removed fail to discover putrid flesh; and E. Hasse has observed
that male humble-bees whose antennæ have been removed cannot discover
the females. The sensory elements are lodged in pits or cones, which may
be filled with liquid, peculiar sensory rods or hairs being associated
with the nerve-endings. Of these pits the queen-bee has, according to
Mr. Cheshire, 1600, the worker 2400, and the drone nearly 19,000, on
each antennæ. On the antennæ of the male cockchafer, Hauser estimates
the number to be 39,000.
In the aquatic crayfish there are, besides the long antennæ, smaller
antennules, each of which has two filaments, an inner and an outer. On
the under surface of most of the joints of the outer filament there are
two bunches of minute, curiously flattened organs, which were regarded
by Leydig, their discoverer, as olfactory. Observation, too, seems to
confirm the view that the sense of smell (or telæsthetic taste) is
located in the antennule. I tried on a crayfish the following
experiment: When it was at rest at the bottom of its tank, I allowed a
current of pure water (the water in which it lived) to flow from a
pipette over its antennæ and antennules. The antennæ moved slowly, but
the antennules remained motionless. I then took some water in which a
cod's head had been boiled, and allowed some of this to stream over the
antennæ and antennules. The former moved slightly as before, but the
antennules were thrown into a rapid up-and-down jerky vibration, and
shortly afterwards the crayfish began moving about the bottom of its
tank. If only one antennule be thus stimulated, or stimulated to a
higher degree than the other, the crayfish seems generally (but not
always) to turn to that side in search of food. Mr. Bateson[EV] has
shown to how large an extent shrimps and prawns seek their food by
smell, and states that a prawn, though blind, will often find his way
back to his proper place, and stay in it.
In the snail the anterior pair of "horns," or tentacles, are said to be
olfactory. Near the end of each is a large ganglion, or nerve-knot, from
which fibres pass to the surface, in which there are said to be
developed sensory knobs. Snails, however, from which these tentacles
have been removed are apparently still possessed of a sense of smell.
Certain lobed processes round the mouth have been regarded as the seat
of olfactory sensation, but this is doubtful. In the foot of the snail,
the part on which it glides, there is a hollow gland, and in this there
are special cells, each of which gives off a delicate rod, enlarging at
the free end into a ciliated knob. These are regarded as sensory and, it
may be, olfactory. In shell-fish like the mussel, in which the water is
sucked in by an inhalent tube or siphon, and ejected through an exhalent
siphon above it (see Fig. 2, p. 4), there is at the entrance of the
incoming current a thin layer of elongated cells which are described as
olfactory, and are in association with a special ganglion. Olfactory
depressions have been described in some worms. But in a great number of
the lower invertebrates very little or nothing is known concerning a
sense of smell.
* * * * *
_Hearing_ is a telæsthetic sense. Through it we become aware of certain
vibratory states of more or less distant objects. The vibrations of
these bodies are transferred to the air or other medium surrounding the
body, and are transmitted through the air or other medium to the ear.
The sound-waves traverse the air at a rate of 337 metres (1106 feet) in
a second; but they travel about four times as fast in water. If the
vibration is periodic or regular, the sound is called a tone;
non-periodic or irregular sounds are noises. The pitch of a tone is
determined by the number of vibrations in a second. The lowest or
gravest tone most of us can hear is that where there are about 30
vibrations in a second; twice this number give us a tone of an octave
higher; twice this again, another octave; and so on. In musical
composition, tones from about 40 to about 4000 vibrations per second are
employed. This is a range of somewhat over six octaves. But many of us
are capable of hearing sounds over a range of about ten octaves, that is
to say, from 30 to 30,000 vibrations per second. The upper limit of
hearing is, however, very variable. Some people are deaf to tones of
more than 15,000 or 20,000 vibrations per second.[EW] Others may hear
shrill tones of 40,000, or even in rare cases 50,000. I could as a boy
hear the shrill squeak of a bat; now I am quite deaf to it. A friend of
mine in South Africa was unable to hear the piping of the frogs in the
pond, which was to me so loud as almost to drown the tones of his voice.
Apart from the pitch of a note is its quality. The same note struck on
different instruments or sung by different persons has a different ring.
This is determined by the number and intensity of overtones, or
partials, which are associated with the fundamental tone. Suppose the
deep fundamental tone of 33 vibrations be sounded; with it there may be
associated overtones, eight or nine in number, all of which are simple
multiples (twice, thrice, four times, and so on) of the fundamental 33.
The effects of these on the organ of hearing fuse or combine with the
predominant effect of the fundamental tone. In harmonious chords, also,
two or more fundamental tones, with their accompaniment of partials,
blend in sensation so completely that it requires a keen musical ear and
some training to analyze them into their component elements.
The delicacy of discrimination of tones is greatest in the mid-region of
hearing; and there is much individual variation in accuracy of ear. I
have made experiments on many individuals to test their powers in this
respect. I found some who were unable, in the mid-region of hearing, to
state which was the higher of two notes sounded on a violin, the tones
of which were separated by a major third, and in one case by a fifth.
With notes on the piano the discrimination was more delicate,
and yet more delicate when the notes were sung. In such cases
tone-discrimination is deficient; and between these and the musician,
who is stated to be able to distinguish tones separated by only 1/64 of
a tone, there are many intermediate stages.
It is beyond my purpose to describe, in more than a very general way,
the nature of the auditory apparatus of man. The vibrations of the air
are received by the drum-membrane, which lies in the auditory passage.
From this it is transmitted, by a chain of small bones, to the inner
auditory apparatus. This consists of two small membranous sacs, with one
of which three membranous looped tubes, the semicircular canals, are
connected; with the other is connected a spiral tube, the cochlear
canal. These membranous sacs and canals are filled with fluid, and are
surrounded by the fluid which fills the bony cavity in which they lie.
This bony cavity has two little windows, one oval and the other round,
across each of which a membrane is stretched. The oval membrane is in
connection with the chain of auditory bones; and when this is made to
vibrate in and out, the membrane of the round window vibrates out and
in. Thus the fluid around and within the membranous sacs and canals is
set in vibration. And the parts are so arranged that the vibrations, in
passing from the oval to the round membrane, must run up one side and
down the other side of the cochlear canal. As they run down they set in
vibration a delicate membrane which is supported on beautiful arched
rods (the organs of Corti). And this membrane contains a number of
special hair-cells, so called because they bear minute hair-like
structures. These are the special end-organs of hearing. It has been
suggested that the fibres of the membrane on the arched rods, which are
of different lengths and may be stretched with differing degrees of
tension, respond to vibrations of different pitch. Thus the hair-cells
on that particular part of the membrane would be stimulated, and the
note might be appreciated in its true position in the scale.
We must now pass on to consider the sense of hearing in animals. That
the mammalia have this sense well developed is a matter of familiar
observation, and in some of them, such as the horse and the deer, it is
exceedingly acute. The form and movements of the external ear also
enable many of the mammalia to collect and attend to sounds from special
directions. The mammalia possess also the power of tone-discrimination,
as is shown by the fact that our domesticated animals recognize
different modulations of the human voice, and that wild creatures
distinguish tones or noises of different quality. A Newfoundland dog,
possessed by a friend of mine, always howled when the tenor D was struck
on the piano, or sung. And Théophile Gautier reports that one of his
cats could not endure the note G, and always put a reproving and
silencing paw on the mouth of any one who sang it.
In birds the sense of hearing is not only very sensitive, but the power
of discrimination is exceedingly delicate. No one who has watched a
thrush listening for worms can doubt that her ear is highly sensitive.
The astonishing accuracy with which many birds imitate, not only the
song of other birds, but such unwonted sounds as the clink of glasses or
the ring of quoits, shows that the delicacy in discrimination has
reached a high level of development. In birds, however, the cochlear
canal has not the same development that it has in mammals, and there are
no arched rods--no organs of Corti.
Nothing special is to be noted concerning the sense of hearing in the
reptiles, amphibia, and fishes. In all (with the exception of the lowly
lancelet) the auditory organ is developed. We shall, however, presently
see reason to question whether the possession of an "auditory organ,"
with well-developed semicircular canals, necessarily indicates the power
of hearing. And Mr. Bateson's recent experiments at Plymouth[EX] seem to
indicate that fishes are not so sensitive in this respect as anglers[EY]
are wont to believe. "The sound made by pebbles rattling inside an
opaque glass tube does not attract or alarm pollack; neither are they
affected by the sharp sound made by letting a hanging stone tap against
an opaque glass plate standing vertically in the water." Carp at Potsdam
are, indeed, said to come to be fed at the sound of a bell. But Mr.
Bateson well remarks that this "can scarcely be taken to prove that the
sound of the bell was heard by them, unless it be clearly proven that
the person about to feed them was hidden from their sight." There is
clearly room for further observation and experiment in this matter.
Turning to the invertebrata, we find, even in creatures as low down in
the scale of life as jelly-fish, around the margin of the umbrella in
certain medusa, simple auditory organs. In some cases they are pits
containing otoliths (minute calcareous or other bodies, which are
supposed to be set a-dance by the sound-vibrations); in others there is
a closed sac with one or more otoliths; in others, again, they are
modified tentacles, partially or completely enclosed in a hood. All
these are generally regarded as auditory, there being specially modified
cells of the nature of hair-cells. We shall see, however, that another
interpretation of organs containing otoliths is at any rate possible.
For the present, we will follow the usual interpretation, and regard
them as auditory.
Vesicular organs containing otoliths are found near the cerebral ganglia
in some of the worms and their relations. But the common earthworm,
though it appears to be sensitive to sound, does not appear to have any
such organs.
Molluscan shell-fish are generally provided with auditory organs. In the
fresh-water mussel it is found in the muscular foot. It can be more
readily seen in the _Cyclas_, if the transparent foot of this small
mollusc be examined under the microscope. It is a small sac containing
an otolith. Mr. Bateson found that the mollusc _Anomia_ "can be made to
shut its shell by smearing the finger on the glass of the tank so as to
make a creaking sound. The animals shut themselves thus when the object
on which they were fixed was hung in the water by a thread." In the
snail and its allies the auditory sac is found in close connection with
the nerve-collar that surrounds the gullet. In the cuttle-fishes it is
found embedded in the cartilage of the head.
[Illustration: Fig. 26.--Antennule of crayfish.
i.j., inner joint; o.j., outer joint; ol., olfactory setæ; ol'., the
same, enlarged; au.op., auditory opening in the basal division, which
has been cut open to show au.s., the auditory sac; au.n., auditory nerve
branching to the two ridges beset with auditory hairs; au.h., auditory
hair, enlarged. (After Howes.)]
[Illustration: Fig. 27.--Diagram of ear.
t.m., tympanic membrane, to which is attached a chain of small bones
stretching across the cavity of the drum, the innermost of which, st.,
fits into the "oval window." The vibrations are transmitted up one side
and down the other side of the cochlear canal, c.c., and thus reach the
"round window," f.r.; s.c. is one of the semicircular canals, the other
two are omitted; e.t. is the Eustachian tube connecting the cavity of
the drum with the mouth-cavity.]
In the lobster or crayfish the auditory organs are found at the base of
the smaller feelers or antennules. They are little sacs formed by an
infolding of the external integument (see Fig. 26, p. 259). Beautifully
feathered auditory hairs project into the sac along specialized ridges,
and the sac in many cases contains grains of sand which play the part of
otoliths. Hensen seems to have proved that shrimps collect the grains of
sand and place them in the auditory sac for this purpose. The curious
shrimp-like _Mysis_ has two beautiful auditory sacs in its tail. These
are provided with auditory hairs. Hensen watched these under the
microscope while a musical scale was sounded, and found that the special
hairs responded each to a certain note. When this particular note was
sounded the hair was thrown into such violent vibration as to become
invisible, but by other notes it was unaffected.
[Illustration: Fig. 28.--Tail of _Mysis_.
au., auditory organ.]
[Illustration: Fig. 29.--Leg of grasshopper.
ty., tympanic membrane.]
Passing now to insects, we may first note that grasshoppers and crickets
have an auditory organ on the front leg. These are provided with
tympanic membranes, and the breathing-tubes, or tracheæ, are so arranged
that the pressure of the air is equalized on the two sides of the
membrane--just as in us and other vertebrates the same end is effected
by a tube which runs from the interior of the drum of the ear to the
mouth-cavity (see Fig. 27). In the organ within the leg there is a group
of cells, followed by a row of similar cells which diminish regularly in
size from above downwards. Each is in connection with a nerve-fibril,
and contains a delicate auditory rod. It has been suggested that the
diminution in size of the cells may have reference to the appreciation
of different notes, but nothing definite is known on the matter. Ants,
too, have an auditory organ, as shown by Sir John Lubbock, in the tibia
of the front leg. But in locusts it is situated on the first segment of
the abdomen. In flies there are a number of vesicles, generally regarded
as auditory (but by some as olfactory), at the base of the rudimentary
hind wings--the so-called halteres, or balancers.
Observation seems to point to the fact that in most insects the sense of
hearing is lodged in the feelers, or antennæ. Kirby made the following
observation on a little moth: "I made," he says, "a quiet, not loud, but
distinct noise; the antenna nearest to me immediately moved towards me.
I repeated the noise at least a dozen times, and it was followed every
time by the same motion of that organ, till at length the insect, being
alarmed, became more agitated and violent in its motions." Hicks wrote,
in 1859, "Whoever has observed a tranquilly proceeding capricorn beetle
which is suddenly surprised by a loud sound, will have seen how
immovably outward it spreads its antennæ, and holds them porrect, as it
were, with great attention, as long as it listens." The same observer
described certain highly specialized organs in the antennæ of the
hymenoptera (ants, bees, and wasps), which he thus describes: "They
consist," he says, "of a small pit leading into a delicate tube, which,
bending towards the base, dilates into an elongated sac having its end
inverted." Of these remarkable organs, Sir John Lubbock says there are
about twelve in the terminal segment, and he has suggested that they may
serve as microscopic stethoscopes.
Mayer, experimenting with the feathered antenna of the male mosquito,
found that some of the hairs were thrown into vigorous vibration when a
note with 512 vibrations per second was sounded. And Sir John Lubbock,
who quotes this observation, adds,[EZ] "It is interesting that the hum
of the female gnat corresponds nearly to this note, and would
consequently set the hairs in vibration." The same writer continues,
"Moreover, those auditory hairs are most affected which are at right
angles to the direction from which the sound comes. Hence, from the
position of the antennæ and the hairs, a sound would act most intensely
if it is directly in front of the head. Suppose, then, a male gnat hears
the hum of a female at some distance. Perhaps the sound affects one
antenna more than the other. He turns his head until the two antennæ are
equally affected, and is thus able to direct his flight straight towards
the female."
It is difficult to determine the range of hearing in the lower
organisms. But it is quite possible, nay, very probable, that the
superior limit of auditory sensation is much more extended in insects
than it is in man. We know that many insects, such as the cicadas, the
crickets and grasshoppers, many beetles, the death's-head moth, the
death-watch, and others, make, in one way or another, sounds audible to
us. But there may be many insect-sounds--we may not call them
voices--which, though beyond our limits of hearing, are nevertheless
audible to insects. At the other end of the scale, on the other hand,
slow pulsations may be appreciated--for example, by aquatic
creatures--by means of what we term the auditory organs, in a way that
is not analogous to the sensation of sound in us. It may be noted that
auditory organs are dotted about the body somewhat promiscuously in the
various invertebrates. We have seen that auditory organs, or what are
generally believed to be such, are found in the foot of bivalves, in the
antennules of lobsters, in the fore legs of crickets and ants, in the
abdomen of locusts, in the balancers of flies, and in the tail of
_Mysis_. But when we come to consider the matter, there is no reason why
the organ of hearing should be in any special part of the body. The
waves of sound rain in upon the organism from all sides. There is no
great advantage in having the organs of hearing in the line of
progression, as with sight, where the rays come in right lines; nor in
having them in close association with the mouth, as in the case of the
organ of smell.
Closely connected with the organ of hearing in vertebrates is the organ
of another and but recently recognized sense. In briefly describing the
auditory apparatus in man, mention was made of three curved membranous
loops, the so-called semicircular canals. A few more words must now be
said about them and the membranous sac with which they are connected.
The sac lies in a somewhat irregular cavity in a bone at the side of the
head, in the walls of which are five openings leading into curved
tunnels in the bone in which lie the membranous loops. The planes in
which the three semicircular canals lie are nearly at right angles to
each other, and they are called respectively the horizontal, the
superior, and the posterior. The two latter unite at one end before they
reach the sac; hence there are five, and not six, openings into the
cavity. At one end of each semicircular canal is a swelling, or ampulla,
in each of which is a ridge, or crest, abundantly supplied with
hair-cells. And in a little recess in the sac there is, occupying its
floor, its front wall, and part of its outer wall, a patch of hair-cells
covered by a gelatinous material with numerous small crystalline
otoliths. The only other point that calls for notice is that the
membranous sac does not fit closely in the bony cavity in which it lies,
while the diameter of the membranous semicircular canals is considerably
less than that of their bony tunnels, except at the ampullæ, or
swellings, where they fit pretty closely. Both the bony cavity and the
membranous labyrinth (as it is called) are filled with fluid.
From its close connection with the organ of hearing, this apparatus was
for long regarded as in some way auditory in its function, and it was
surmised that it enabled us to perceive the direction from which the
sound came. But how it could do so was not clear. In 1820 M. Flourens
made the observation that the injury or division of a membranous canal
gave rise in the patient to rotatory movements of the animal round an
axis at right angles to the plane of the divided canal; and he,
therefore, suggested that the canals might be concerned in the
co-ordination of movement. They are now regarded as the organs of a
sense of rotation or acceleration.
That we have such a sense of rotation has been proved
experimentally.[FA] Let a man, blindfolded, sit on a smooth-running
turn-table. When it begins to rotate he feels that he is being moved
round, but if the rotation be continued at the same rate, this feeling
quickly dies away. If the rotation be increased, he again feels as if he
were being moved round, but this again soon dies away. Further increase
gives a fresh sensation, which in turn subsides, and the man may then be
spinning round rapidly, and be perfectly unconscious of the fact. He is
only aware that he has been gently turned round a little two or three
times. Now let the speed of rotation be slackened. He has a sensation of
being gently turned round a little in the opposite direction. Each time
the speed is lessened he has this sense of being turned the reverse way.
From these experiments we see that what we are conscious of is change of
rate of rotation, or, in technical language, acceleration, positive or
negative.
[Illustration: Fig. 30.--Diagram of semicircular canals.
A. bony labyrinth of human ear (after Sömmering). c, c., the cochlea;
s.c., superior semicircular canal; p.c., posterior semicircular canal;
h.c., horizontal semicircular canal; a, a, a, their swellings, or
ampullæ; f.o., f.r., fenestra ovalis and rotunda (oval and round
windows) in the vestibule.
B. Diagram of semicircular canal to illustrate effect of rotation. The
large arrows indicate the direction of the rotation. The small arrow to
the left indicates the resulting flow of the inner fluid into the
ampulla; that to the right, the flow of the outer fluid into the
vestibule.]
From Professor Crum Brown's paper in_ Nature_ I transcribe, with some
verbal modifications, his account of how the semicircular canals enable
us to feel these changes of motion. Let us consider the action of one
canal. If the head be rotated about a line at right angles to the plane
of the canal, with the ampulla leading, there will be a tendency for the
fluid within the sac to flow into the ampulla, and for the fluid around
the semicircular canal to flow into the cavity in which the sac lies.
These movements will conspire to stretch the membranous ampulla, and
thus to stimulate the hair-cells. This stretching will not take place in
that canal if the rotation be in the reverse direction. But on the
opposite side of the head is another canal in the same plane, but turned
the other way. In the reversed rotation the ampulla in this canal will
lead, and its hair-cells will be stimulated. Thus by means of the two
canals on either side of the head in the same plane, rotation in either
direction can be appreciated. And since there are two other pairs of
semicircular canals in two other planes, rotation in any direction will
be recognized by means of one or more of the six canals.
It is thus by means of the semicircular canals that we can appreciate
acceleration of rotatory motion.[FB] But we can also appreciate
acceleration of movements of translation--forwards or backwards, up or
down. And Professor Mach has suggested that it is through the
stimulation of the hair-cells in the patch in the sac itself (the
so-called _macula acustica_) that we are able to appreciate these
changes. The otoliths, held loosely and lightly in position by the
gelatinous substance in which they are embedded, may, through their
inertia, aid in the stimulation of the sense-hairs.
And this naturally suggests the question whether those sense-organs in
the invertebrates which contain otoliths may not be regarded with more
probability as organs for the appreciation of changes of motion than as
auditory organs. This for some years has been my own belief. I have
always felt a difficulty in understanding how the otoliths are set
a-dance by auditory vibrations. But their inertia would materially aid
in the appreciation of changes of motion. In some forms the otoliths are
held in suspension in a gelatinous material. In others--the molluscs,
for example--the otolith (which is generally single) is retained in a
free position by ciliary action. In aquatic creatures an organ for the
appreciation of changes of motion might be of more service than an
auditory organ. And if one be permitted to speculate, one may surmise
that the sense of hearing may be a refinement of the sense through which
changes of motion are appreciated. First would come a sense of movements
of the organism in the medium through the stimulation of the sense-hairs
by the relative motion of the otolith; then these sense-hairs, with
increased delicacy, might appreciate shocks in the medium; and,
eventually, those more delicate shocks which we know as auditory waves.
In this way we might account for the fact that in the vertebrates the
same organ, through different parts of its structure, appreciates both
change of motion and auditory vibrations. And thus the organs in the
invertebrata which are generally regarded as auditory, and for which has
been suggested the function of reacting to changes of motion, may, in
truth, subserve both purposes--may be organs in which the
differentiation I have hinted at is taking place.
* * * * *
_Sight_, like hearing, is a telæsthetic sense. Through it we become
aware of certain vibratory states of more or less distant objects. The
medium by means of which these vibrations are transmitted is not, as in
the case of hearing, the air, but the æther which pervades all space.
The rate of transmission is about 186,000 miles in a second. That which
answers in vision to pitch in hearing is colour. The lowest, or gravest,
light-tone to which we are sensitive is deep red, where the number of
vibrations per second is about 370 billions (370,000,000,000,000). The
highest, or most acute, light-tone is violet, with about 833 billion
vibrations in a second. If white light be passed through a prism, the
rays are classified according to their vibration-periods, and are spread
out in a spectrum, or band of rainbow colours. But different individuals
vary, as we shall presently see, in their sensibility to the lowest and
the highest vibrations. Some people are, moreover, relatively or
absolutely insensible to certain colours, generally either red or green.
Such persons are said to be colour-blind. When the rainbow colours are
combined in due proportion, or when pairs or sets of them are combined
in certain ways, white light is produced.
We saw that in the case of sound-waves, when the number of vibrations in
a second is doubled, the sound is raised in pitch by an octave. Using
this term in an analogous way for colour-tones, we may say the range in
average vision is about one octave--that is, from about 400 billion to
about 800 billion vibrations in a second. But, though these are the
limits in human vision, we know of the existence of many octaves of
radiant energy physically in continuity with the light-vibrations.
Photography has made us acquainted with ultra-violet vibrations up to
about 1600 billions per second--an octave above the violet. And
Professor Langley's observations with the bolometer indicate the
existence of waves with as low a vibration-period as one billion per
second, and even here, in all probability, the limit has not been
reached. To the vibrations more rapid than those that are concerned in
the sensation of violet, the human organism is apparently in no manner
sensitive. But to infra-red vibrations down to about thirty billions per
second the nerves of the skin respond through the temperature-sense. We
shall have to return to these limits of sensation at the close of this
chapter.
[Illustration: Fig. 31.--The human eye. Horizontal section, to show
general structure.]
[Illustration: Fig. 32.--Retina of the eye. Enlarged section of minute
fragment.
b., back of retina next the outer coat; l.r.c., layer of rods and cones;
i.l., intermediate layers; l.g.c., layer of ganglion-cells; l.n.f.,
layer of nerve-fibres; f., front of retina, the surface turned towards
the pupil.]
The human eye is a nearly spherical organ, capable of tolerably free
movements of rotation in its socket. What we may call the outer case,
which is white and opaque elsewhere, is quite transparent in front.
Through this transparent window may be seen the coloured iris, in the
centre of which is a circular aperture, the pupil. The size of the pupil
changes with the amount of light--it dilates or contracts, according as
the light is less or more intense. Just behind it, and still in the
front part of the eye, is the transparent lens, the convexity of the
anterior surface of which can be altered in the accommodation of the
organ for near or far vision. The space between the lens and iris and
the corneal window of the eye is filled with a watery fluid. Behind the
lens there is a transparent, semi-fluid, jelly-like material, filling
the rest of the chamber of the eye. At the back of the eye is spread out
the sensitive membrane--the retina. The structure of this membrane is
very complicated, and cannot be described here. It is, however,
indicated in Fig. 32. For our present purpose it is sufficient to note
that here are the end-organs of the optic nerve; that these consist of a
number of delicate rods and cones; and that these rods and cones do not
face in the direction from which the light comes, but face towards the
back of the eyeball, where a pigmented substance is developed. The rays
of light are thus focussed through the retina on to this pigmented
substance; the ends of the rods and cones are stimulated; and the
stimulation is handed on, augmented in certain intermediate ganglia, to
the delicate transparent nerve-fibres in the front of the retina. These
collect to a certain spot, where they pass through the retina to form
the optic nerve. Where they pass through the retina there can, of
course, be no rods and cones. And in this spot there is no power of
vision. It is the blind spot. The reality of its existence can easily be
proved. Make a dot on a piece of writing-paper, and about three inches
to the left of it place a threepenny or sixpenny bit. Close the right
eye, and look with the left eye at the dot. The sixpenny bit will also
be seen, but not distinctly. Keep the eye fixed on the dot, and move the
head slowly away from the paper. At a distance of about ten inches the
coin will completely disappear from view. Its image then falls on the
blind spot.
The organ of vision, then, in us consists of an essential sensory
membrane, the retina, with its delicate rods and cones; and an accessory
apparatus for focussing an inverted image on to the sensitive surface of
the retina. The surface is not, however, equally sensitive, or, in any
case, does not give an equal power of discrimination, throughout its
whole extent. This is seen in the experiment above described. When we
look at the dot we see the coin, but not distinctly. The area of clear
and distinct vision is, in fact, very small, constituting the yellow
spot about 1/12 of an inch (2 millimetres) long, and 1/30 of an inch (.8
millimetre) broad. And even within this small area there is a still more
restricted area of most acute sensibility only 1/120 of an inch (.2
millimetre) in diameter. Nevertheless, within this minute area there are
some two thousand cones, the rods being here absent. In carefully
examining an object we allow this area of acute vision to range over it.
Hence the extreme value of that delicate mobility which the eye
possesses--a mobility that is accompanied by muscular sensations of
great nicety.
We saw that the sense of touch in the tongue is sufficiently delicate to
enable us to recognize, as two, points of contact separated by 1/25 of
an inch (1.1 millimetre). What, in similar terms, is the delicacy of
sight? At what distance apart, on the most delicate part of the retina,
can two points of stimulation be recognized as distinct from each other?
If the points of stimulation be not less than 1/6000 of an inch (.004
millimetre) apart, they can be distinguished as two. Below this they
fuse into one. The diameter of the end of a single cone in the yellow
spot is also about 1/6000 of an inch (.0045 millimetre).
With regard to the mode in which the stimulation of the retinal elements
is effected, we have no complete knowledge. Certain observations of Boll
and Kühne, however, show that when an animal is killed in the dark the
retina has a peculiar purple colour which is at once destroyed if the
retina be exposed to light. If a rabbit be killed at the moment when the
image, say, of a window, is formed on the retina, and the membrane at
once plunged in a solution of alum, the image may be fixed, and an
"optogram" of the window may be seen on the retina. The discharge of the
colour of the retinal purple may be regarded as the sign of a chemical
change effected by the impact of the light-vibrations. But in the yellow
spot there seems to be no visual purple. It is, indeed, developed only
in the rods, not in the cones. Here, probably, chemical or metabolic
changes occur without the obvious sign of the bleaching of retinal
purple. In the dusk-loving owl the retinal purple is well developed, but
in the bat it is said to be absent.
We saw that in the case of hearing the auditory organ is fitted to
respond to air-borne vibrations varying from about thirty to thirty
thousand per second. And though the details of the process are at
present not well understood, it is believed that certain parts of the
recipient surface are fitted to respond to low tones, other parts to
intermediate tones, and yet others to high tones. Thus the reception is
serial. If there be two pianos near each other, accurately in tune, any
note struck on one will set the corresponding note vibrating in the
other.[FC] The auditory organ may be likened to this second piano.
Special parts respond to special tones.
Now, in the case of vision, the conditions are different. The reception
cannot be serial. As I range my eye over a flower-bed, I bring the area
of distinct vision on to a number of different colours, and these are
seen to be distinct, though they are received on the same part of the
retinal surface. It might, perhaps, be suggested that special cones were
set apart for each shade of colour. But there are only some two thousand
cones in the central area of most acute vision, and Lyons
silk-manufacturers prepare pattern cards containing as many shades of
coloured silks. So that there would be only one cone to each colour. And
Herschel thought that the workers on the mosaics of the Vatican could
distinguish at least thirty thousand different shades of colour! There
are also many phenomena of colour-blending which show that
colour-reception cannot in any sense be serial.
How, then, are we to account for our wide range of colour-sensation?
Just as the blending by the artist on his palette of a limited number of
pigments gives him the wide range of colour seen on his canvas, so the
blending of a few colour-tones may give us the many shades we are able
to distinguish. The smallest number of fundamental colour-tones which
will fairly well account for the phenomena of colour-vision, is three.
And these three are red, green, and blue or violet. These are the three
so-called primary colours. All others are produced from these elements
by blending.
To explain our ability to appreciate differences of colour, then, it is
supposed, on the hypothesis of Young and Von Helmholtz, that three kinds
of nerve-fibres exist in the retina, the stimulation of which gives
respectively, red, green, and violet in consciousness. Professor
McKendrick, interpreting Von Helmholtz, gives[FD] the following
scheme:--
"1. Red excites strongly the fibres sensitive to red, and feebly the
other two.
"2. Yellow excites moderately the fibres sensitive to red and green,
feebly the violet.
"3. Green excites strongly the fibres sensitive to green, feebly the
other two.
"4. Blue excites moderately the fibres sensitive to green and violet,
feebly the red.
"5. Violet excites strongly the fibres sensitive to violet, feebly the
other two.
"6. When the excitation is nearly equal for the three kinds of fibres,
the sensation is white."
This theory cannot be regarded as more than a provisional hypothesis.
Still, by its means we can explain many colour-phenomena. It is well
known, for example, that if we gaze steadily at a red object, and then
look aside at a grey surface, an after-image of the object will be seen
of a blue colour. According to the theory, the red fibres have been
tired and cannot so readily answer to stimulation. Over this part of the
retina, therefore, the effect of grey light is to stimulate normally the
fibres sensitive to green and violet, but only slightly those sensitive
to red, owing to their tired condition. The result will be, as we see
from the above scheme (4), the sensation of blue. Colour-blind people,
on this view, are those in whom one set of the fibres, generally the red
or the green, are lacking or ill developed.
We may, perhaps, with advantage restate this theory in terms of chemical
change, or metabolism. On this view three kinds of "explosives" are
developed in the retinal cones; for it is seemingly the cones, rather
than the rods, which are concerned in colour-vision. All three explosive
substances are unstable; but one, which we may call R., is especially
unstable for the longer waves of the spectrum; another, G., for the
waves of mid-period; a third, V., for those of smallest wave-length.
Suppose that R. only were developed. If, then, we were to look at a band
of light spread out in spectrum wave-lengths, we should see a band[FE]
of monochromatic _r_. light. Its centre would be bright, and here would
be the maximum instability of R. On either side it would fade away. The
lateral edges of the spectrum would be the limits of the instability of
R. If G. only were developed, we should see only a band of monochromatic
_g._ light. Its centre would not coincide with that for R., but would
lie in a region of smaller wave-length. Here would be the maximum
instability for G. On either side the green would fade away. Its lateral
edges would mark the limits of the instability of G. But though their
centres would not coincide, the R. band and the G. band would to a large
extent overlap. Similarly with the band for V. It, too, would have its
centre of maximum instability and its lateral edges of lessening
instability. Its centre would lie in a region of yet smaller wave-length
than that for G. And the _v._ band would overlap the green and the red.
Normally, all three bands are developed, and their blended overlapping
gives the colours of the rainbow. For this reason the monochromatic
bands _r._, _g._, and _v._ are unknown to us in experience. All the
colour-tints we know are blended tints. What we call full-red light
causes strong disruptive change in R., but decomposes slightly G., and
probably also, but in much less degree, V.
Whether R., G., and V. are all three present in each cone, or whether
they are each developed in separate cones, we do not know for certain.
Nor are we certain that there are separate nerve-fibres for the
transmission of stimuli due to R., G., and V.
When we look steadily at a red object we cause the disruption of R.; and
since it takes some time for the reformation and reconstitution of this
explosive substance, on turning the eye to a grey surface, G. and V. are
alone, or in preponderating proportions, caused to undergo disruption.
Hence the phenomena of complementary after-images. It is not merely a
matter of the tiring of certain nerve-fibres, but a using-up of the
explosive material in certain of the cones.
What is called _colour-blindness_ is probably due to one of several
abnormal conditions. It is _possible_ that in some cases R., G., or V.
may be entirely absent. More frequently they are in abnormal
proportions. They probably vary in their sensitiveness, and not
improbably in the wave-period to which they show the maximum response.
To test the variation, if any, in the limits of instability for R. and
V., or in any case in the limits of colour-vision at the red end and at
the violet end of the spectrum, in apparently normal individuals, my
friend and colleague, Mr. A. P. Chattock, made, at my suggestion, a
number of observations on some of the students of the University
College, Bristol, to whom my best thanks are due for their kind
willingness to be submitted to experiment. The instrument used[FF] was a
single-prism spectro-goniometer.
In the accompanying diagram (Fig. 33) the results of some of these
observations are graphically shown. The middle part of the spectrum,
between the wave-lengths 420 and 740 millionths of a millimetre, is
omitted, only the red end and the violet end being shown. The
observations on thirty-four individuals, seventeen men and seventeen
women, all under thirty years of age, are given for both eyes. The
left-hand vertical line of each pair stands for the right eye in each
case. To the left of the table are placed the wave-lengths in millionths
of a millimetre.
[Illustration: Fig. 33.--Variation in the limits of colour-vision.]
Take, for example, the first pair of vertical lines. The individual
whose colour-range they represent could detect red light in the spectrum
up to 800 millionths of a millimetre wave-length for the right eye, and
up to 811 for the left; and could detect violet light down to 403 and
404. Beyond these limits all was dark. But the last individual in the
series, while his range in the violet was about the same, could only
detect red light up to 743 and 750 millionths of a millimetre. His
spectrum was so much shorter.
It is seen that there is more variation at the red end than at the
violet end of the spectrum, and this notwithstanding that the violet
rays are more spread out by the prism than the red rays. It is seen that
the two eyes are often markedly different. This is not due to inaccuracy
of observation, for certain individuals in which this occurred were
tested several times with similar results. It is seen that the
variations at the red end and the violet end are often independent, and
that the absolute length of the visible spectrum differs in different
individuals.
The following table presents these observations and a few others in
another light:--
Table of Maxima and Minima in Wave-lengths, expressed In
Millionths of a Millimetre.
-------------------------------------------------------------------------
| Violet | Red |
|----------------------+----------------------| No. of
|Highest| Mean | Lowest|Highest| Mean |Lowest|Individuals
|-------+------+-------+-------+-------+------+-----------
Women under 30 | 410.0 |402.75| 394.0 | 811 | 772.85| 743 | 17
Men " " | 413.0 |405.0 | 399.0 | 811 | 772.8 | 743 | 17
Women over 30 | 410.5 |406.65| 401.5 | 792 | 767.8 | 743 | 7
Men " " | 407.0 |404.5 | 402.5 | 787 | 773.7 | 758 | 3
---------------------------------------+---------------------------------
N { right eye | 406 | 687
{ left eye | 407 | 717
-------------------------------------------------------------------------
The individual N showed signs of colour-blindness, and is therefore not
included in the table, but entered separately. He was unable to
recognize the C line of the hydrogen spectrum (wave-length 656), which
was brilliantly obvious to the normal eye.
These observations[FG] need further confirmation and extension. We
intend to continue the investigation each session. They are, however,
sufficient to show that in some individuals R. undergoes disruptive
change on the impact of light-waves which have no noticeable effect on
the retina of other individuals.
It is impossible here to do more than just touch the fringe of the
difficult subject of colour-vision. And the only further fact that can
here be noticed is that trichromatic colour-vision is apparently in us
limited to the yellow-spot and its immediate neighbourhood. Around this
is an area which is said to be bichromatic--all of us being, for this
area, more or less green-blind. In the peripheral area around this,
colour is indistinguishable, and we are only sensitive to light and
shade. So far as the structure of the retina is concerned, we may notice
in this connection that in the central region of most complete
trichromatic vision there are cones only; around the yellow spot each
cone is surrounded by a circle of rods; and further out into the
peripheral region by two, three, or more circles of rods.
Concerning the sense of sight in the lower mammals little need be said.
In many cases the acuteness of vision is remarkable. Mr. Romanes's
experiments on Sally, the bald-headed chimpanzee at Regent's Park, led
him to conclude that she was colour-blind, but I question whether the
experiments described quite justify this conclusion. Sir John Lubbock
was unable to teach his intelligent dog Van to distinguish between
coloured cards; but the failure was as complete when the cards were
marked respectively with one, two, or three dark bands. We are not
justified, therefore, in ascribing the failure to colour-blindness. The
real failure, probably, was in each case to make the animal understand
what was wanted. Bulls are, at any rate, credited with strong
colour-antipathies, and insect-eating mammals are probably not defective
in the colour-sense.
It is said that nocturnal animals, such as mice, bats, and hedgehogs,
have no retinal cones; and if the cones are associated with
colour-vision, they may not improbably be unable to distinguish colours.
Some moles are blind (e.g. the Cape golden mole). But the common
European mole, though the eyes are exceedingly minute (1/25 of an inch
in diameter), has the organ fairly developed, and is even said not to be
very short-sighted. It is protected by long hairs when the animal is
burrowing, and is only used when it comes to the surface of the ground.
It is probably in birds that vision reaches its maximum of acuteness. A
tame jackdaw will show signs of uneasiness when seemingly nothing is
visible in the sky. Presently, far up, a mere speck in the blue, a hawk
will come within the range of far-sighted human vision. Steadily watch
the speck as the hawk soars past, until it ceases to be visible; the
jackdaw will still keep casting his eye anxiously upward for some little
time. He may be only watching for the possible reappearance of the hawk.
But just as he saw it before man could see it, so probably he still
watches it after, to man's sight, it has become invisible. So, too, for
nearer minute objects, the swift, as it wheels through the summer air,
presumably sees the minute insects which constitute its food. And every
one must have noticed how domestic fowls will pick out from among the
sand-grains almost infinitesimal crumbs.
It is probable that the area of acute vision is much more widely
diffused over the retina of birds than it is with us. In any case, the
cones are more uniformly and more abundantly distributed over the
general retinal surface.
An exceedingly interesting and important peculiarity in the retina of
birds, which they share with some reptiles and fishes, is the
development, in the cones, of coloured globules. "The retinæ of many
birds, especially of the finch, the pigeon, and the domestic fowl, have
been carefully examined by Dr. Waelchli, who finds that near the centre
green is the predominant colour of the cones, while among the green
cones red and orange ones are somewhat sparingly interspersed, and are
nearly always arranged alternately--a red cone between two orange ones,
and _vice versâ_. In a surrounding portion, called by Dr. Waelchli the
red zone, the red and orange cones are arranged in chains, and are
larger and more numerous than near the yellow spot; the green ones are
of smaller size, and fill up the interspaces. Near the periphery the
cones are scattered, the three colours about equally numerous and of
equal size, while a few colourless cones are also seen. Dr. Waelchli
examined the optical properties of the coloured cones by means of the
micro-spectroscope, and found, as the colours would lead us to suppose,
that they transmitted only the corresponding portions of the spectrum;
and it would almost seem, excepting for the few colourless cones at the
peripheral part of the retina, that the birds examined must have been
unable to see blue, the whole of which would be absorbed by their
colour-globules."[FH]
These facts are of exceeding interest. They seem to show that for these
birds the retinal explosives are not the same as for us. They are R.,
O., and G. Moreover, the colour-globules will have the effect of
excluding the phenomena of overlapping. For each kind of cone the
spectrum must be limited to the narrow spectral band transmissible
through the associated colour-globule. If these facts be so, it is not
too much to say that the colour-vision of birds must be so utterly
different from that of human beings, that, being human beings, we are
and must remain unable to conceive its nature. The factors being
different, and the blending of the factors by overlap being, by
specially developed structures, lessened or excluded, the whole set of
resulting phenomena must be different from ours. And this is a fact of
the utmost importance when we consider the phenomena of sexual selection
among birds, and those theories of coloration in insects which involve a
colour-sense in birds.
Concerning the sense of sight in reptiles and in amphibians, little need
here be said. At near distances some of them undoubtedly have great
accuracy of vision. This is, perhaps, best seen in the chamæleon. In
this curious animal the eyes are conical, and each moves freely,
independently of the other. The eyelids encase the organ, except for a
minute opening, looking like a small ink-spot at the blunted apex of the
cone. The animal catches the insects on which it feeds by darting on to
them its long elastic tongue and slinging them back into the mouth,
glued to its sticky tip. Its aim is unerring, but it never strikes until
both eyes come to rest on the prey, and great accuracy of vision must
accompany the great accuracy of aim. Frogs and toads capture their prey
in a somewhat similar way; and a great number of reptiles and amphibians
are absolutely dependent for their subsistence on the acuteness and
accuracy of their vision, which is, however, on the whole, markedly
inferior to that of birds.
In fishes, from their aquatic habit, the lens and dioptric apparatus are
specially modified, in accordance with the denser medium in which they
live; and one curious fish, the Surinam sprat, is stated to have the
upper part of the lens suited for aerial, and the lower part for aquatic
vision.
Mr. Bateson[FI] has made some interesting observations on the sense of
sight in fishes. He finds that in the great majority of fishes the shape
and size of the pupil do not alter materially in accordance with the
intensity of the light. The chief exceptions are among the Elasmobranchs
(dog-fishes and skates). In the torpedo the lower limb of the iris rises
so as almost to close the pupil, leaving a horizontal slit at the upper
part of the eye. In the rough dog-fish, the angel-fish, and the
nurse-hound, the pupil closes by day, forming merely an oblique slit. In
the skate a fern-like process descends from the upper limb of the iris.
The contraction in these cases does not seem to take place rapidly as in
land vertebrates, but slowly and gradually.
Among diurnal fishes belonging to the group of the bony fishes
(Teleosteans), the turbot, the brill, and the weever have a semicircular
flap from the upper edge of the iris, which partially covers the pupil
by day, but is almost wholly retracted at night.
None of the fishes observed by Mr. Bateson appears to distinguish food
(worms) at a greater horizontal distance than about four feet, and for
most of them the vertical limit seemed to be about three feet; but the
plaice at the bottom of the tank perceived worms when at the surface of
the water, being about five feet above them. Most of them exhibited
little power of seeing an object below them. But though the distance of
clear vision seems to be so short for small objects in the water, many
of these fish (plaice, mullet, bream) notice a man on the other side of
the room, distant about fifteen feet from the window of the tank. The
sight of some fishes, such as the wrasses (_Labridæ_), is admirably
adapted for vision at very close quarters. "I have often seen," says Mr.
Bateson, "a large wrasse search the sand for shrimps, turning sideways,
and looking with either eye independently, like a chamæleon. Its vision
is so good that it can see a shrimp with certainty when the whole body
is buried in grey sand excepting the antennæ and antenna-plates. It
should be borne in mind that, if the sand be fine, a shrimp will bury
itself absolutely, digging with its swimmerets, kicking the sand
forwards with its chelæ, finally raking the sand over its back, and
gently levelling it with its antennæ; but if the least bit be exposed,
the wrasses will find it in spite of its protective coloration."
* * * * *
[Illustration: Fig. 34.--Pineal eye.
Modified eye-scale of a small lizard, _Varanus benekalensis_. (After
Baldwin Spencer.)]
Although it is probably not functional in any existing form, mention
must here be made of the median or pineal eye. On the head of the common
slow-worm, or blind-worm, there is a dark patch surrounding a brighter
spot. This is the remnant of a median eye. It has been found in varying
states of degeneration in many reptiles (Fig. 34), and in a yet more
vestigial form in some fishes and amphibia. It is connected with a
curious structure, associated with the brain of all vertebrates, and
called the pineal gland. Descartes thought that this was the seat of the
soul; but modern investigation shows it to be a structure which has
resulted from the degeneration of that part of the brain which was
connected with the median eye. There is some reason to suppose that, in
ancient life-forms, like the Ichthyosaurus, and Plesiosaurus, and the
Labyrinthodont amphibians, it was large and functional. In any case,
there is a large hole in the skull (Fig. 35) through which the nervous
connection with the brain may have been established. The structure of
the eye is not similar to that of the lateral eye, but more like that of
some of the invertebrates.
To these invertebrates we must now turn.
[Illustration: Fig. 35.--Skull of _Melanerpeton_.
A Labyrinthodont amphibian from the Permian of Bohemia (after Fritsch).
× 4. Pa., the parietal foramen.]
* * * * *
Insects have eyes of two kinds. If we examine with a lens the head of a
bee, we shall see, on either side, the large compound or facetted eye;
but in addition to these there is on the forehead or vertex a triangle
of three small, bright, simple eyes, or ocelli. These ocelli, or
eyelets, differ, in different insects, as to the details of their
structure; but in general they consist of a lens produced by the
thickening of the integumentary layer which is at the same time rendered
transparent. Behind this lies the so-called vitreous body, composed of
transparent cells, and then follows the retina, in which there are a
number of rods, the recipient ends of which are turned towards the rays
of light, and not away from them as in the vertebrate. Spiders have from
six to eight ocelli, arranged in a pattern on the top of the head.
Facetted eyes are not found in them.
[Illustration: Fig. 36.--Eyes and eyelets of bee.
A. Drone. B. Worker.]
These facetted eyes, which are found in both insects and crustacea, have
apparently a more complex structure than the ocelli. Externally--in the
bee, for example--the surface is seen to be divided up into a great
number of hexagonal areas, each of which is called a facet, and forms
(in some insects, but not in all) a little lens. Of these the queen bee
has on each side nearly five thousand; the worker some six thousand; and
the drone upwards of twelve thousand; while a dragon-fly (_Æschna_) is
stated to have twenty thousand. Beneath each facet (in transverse
section, Fig. 37) is a crystalline cone, its base applied to the lens,
its apex embraced by a group of elongated cells, in the midst of which
is a nerve-rod which is stated to be in direct connection with the
fibres of the optic nerve. Dark pigment is developed around the
crystalline cones. And retinal purple is said to be present in the cells
which underlie it.
With regard to these facetted eyes there has been much discussion. The
question is--Is each facetted organ an eye, or is it an aggregate of
eyes? To this question the older naturalists answered confidently--An
aggregate. A simple experiment seems to warrant this conclusion. If the
facetted surface be cleared of its internal structures (the crystalline
cones, etc.) and placed under the microscope, each lens may, at a
suitable distance of the object-glass, be made to give a separate image
of such an object as a candle reflected in the mirror of the microscope.
If each lens thus gives an image, is not each the focussing apparatus of
a single eye? But a somewhat more difficult experiment points in another
direction. If the facetted cornea be removed _with the crystalline cones
still attached_ (Grenacher was able to do it with a moth's eye), and
placed under the microscope, when the instrument is focussed at the
point of the cone (where the nerve-rod comes), a spot of light, and not
an image, is seen. No image can be seen unless the microscope be
focussed for the centre of the cone; and here there are no structures
capable of receiving it and transmitting corresponding waves of change
to the "brain."
[Illustration: Fig. 37.--Eye of fly.
Transverse section through head. (After Hickson.)]
But what, it may be asked, can be the purpose of an eye-structure which
gives, not an image, but merely a spot of light? The answer to this
question can only be found when it is remembered that there are
thousands of these facets and cones giving thousands of spots of light.
The somewhat divergent cones and facets of the insect's eye (Fig. 37)
embrace, as a whole, an extended field of vision; each has its special
point in that field; and each conveys to the nerve-rod which lies
beneath it a stimulation in accordance with the brightness, or
intensity, or quality of that special point of the field to which it is
directed. The external field of vision is thus reproduced in miniature
mosaic at the points of the crystalline cones--thus there is produced by
the juxtaposition of contiguous points a stippled image. And it must be
remembered that, even in human vision, the stimulation is not that of a
continuum, but is stippled with the fine stippling of the ends of the
rods and cones. In insect-vision the stippling is far coarser, and the
image is produced on different principles.
[Illustration: Fig. 38.--Diagram of mosaic vision.]
In the vertebrate the image is produced by a lens; in the insect's eye,
by the elongated cones. How this is effected will be readily seen with
the aid of the diagram. At _a b_ are a number of transparent rods,
separated by pigmented material absorbent of light. They represent the
crystalline cones. At _c d_ is an arrow placed in front of them; at _e
f_ is a screen placed behind them. Rays of light start in all directions
from any point, _c_, of the arrow; but of these only that which passes
straight down one of the transparent rods reaches the screen. Those
which pass obliquely into other rods are absorbed by the pigmented
material. Similarly with rays starting from any other point of the
arrow. Only those which, in each case, pass straight down one of the
rods reach the screen. Thus there is produced a reduced stippled image,
_c' d'_, of the arrow.
There has been a good deal of discussion as to the relative functions of
the ocelli and the facetted eyes of insects. The view generally held is
that the ocelli are specially useful in dark places and for near vision;
while the facetted eyes are for more distant sight and for the
ascertainment of space-relations. How the two sets of impressions are
correlated and co-ordinated in insect-consciousness, who can say?[FJ]
The interesting observations of Sir John Lubbock seem to show that
insects can distinguish between different colours. "Amongst other
experiments," he says,[FK] "I brought a bee to some honey which I placed
on a slip of glass laid on blue paper, and about three feet off I placed
a similar drop of honey on orange paper. With a drop of honey before her
a bee takes two or three minutes to fill herself, then flies away,
stores up the honey, and returns for more. My hives were about two
hundred yards from the window, and the bees were absent about three
minutes or even less. After the bee had returned twice, I transposed the
papers; but she returned to the honey on the blue paper. I allowed her
to continue this for some time, and then again transposed the papers.
She returned to the old spot, and was just going to alight, when she
observed the change of colour, pulled herself up, and without a moment's
hesitation darted off to the blue. No one who saw her at that moment
could have the slightest doubt about her perceiving the difference
between the two colours."
Passing now to the crustacea, we find in them eyes of the same type as
in insects; but in the higher crustacea ocelli are absent. In the crabs
and lobsters the eyes are seated on little movable pedestals; in the
former the crystalline cones are very long, in the latter they are
short. There can be little doubt that vision is by no means wanting in
acuteness in an animal which, like the lobster, can dart into a small
hole in the rocks with unerring aim from a considerable distance. The
experiments of Sir John Lubbock have shown that the little water-flea
(_Daphnia_) can distinguish differences of colour, yellows and greens
being preferred to blues or reds.
Among the molluscs there are great differences in the power of sight.
Most bivalves, like the mussel, are blind. Interesting stages in the
development of the eye may be seen in such forms as the limpet,
_Trochus_ and _Murex_. The limpet has simply an optic pit, the _Trochus_
a pit nearly closed at the orifice and filled with a vitreous mass, and
the _Murex_ a spherical organ completely closed in with a definite lens.
The snail has a well-developed eye on the hinder and longer horn or
tentacle. But it does not seem to be aware of the presence of an object
until it is brought within a quarter of an inch or less of the tentacle.
In all probability the eye does little more than enable the snail to
distinguish between light and dark. And the same may be said of the eye
of many of the molluscs. In some, however, the cuttle-fishes and their
allies, the eye is so highly developed that it has been compared with
that of the vertebrate. There is an iris with a contractile pupil. And
the ganglion with which it is connected forms a large part of the
so-called brain. The powers of accurate vision in these higher forms are
probably considerable.
It is interesting to note that whereas in the cuttle-fishes and most
molluscs, the rods of the retina are turned towards the light, in
_Pecten_, _Onchidium_ (a kind of slug), and some others, they are, as in
vertebrates, turned from the light. In _Pecten_ the nerve to supply the
retina bends round its edge at one side. But in _Onchidium_ it pierces
the retina as in vertebrates.
In worms, eyes are sometimes present, sometimes absent. In star-fishes
and their allies they often occur. In medusæ (jelly-fish) they are
sometimes found on the margin of the umbrella. Even in lowly organisms,
like the infusoria, eye-spots not unfrequently occur. We must remember,
however, that, in these lower forms of life, the organs spoken of as
eyes or eye-spots merely enable the possessor to distinguish light from
darkness.
Even when eyes or eye-spots are not developed, the organism seems to be
in some cases sensitive to light--using the word "sensitive," once more,
in its merely physical acceptation. The earthworm, for example, though
it has no eyes, is distinctly sensitive to light; and the same has been
shown to be the case with other eyeless organisms. Graber holds that his
experiments demonstrate that the eyeless earthworm can distinguish
between different colours--in other words, is differentially sensitive
to light-waves of different vibration-period--preferring red to blue or
green, and green to blue. And the same observer has shown that animals
provided with eyes--the newt, for example--can distinguish between light
and darkness by the general surface of the skin. M. Dubois, by a number
of experiments on the blind _Proteus_ of the grottoes of Carniola, has
shown that the sensitiveness of its skin to light is about half that of
its rudimentary eyes; and, further, that this sensibility varies with
the colour of the light employed, being greatest for yellow light.[FL]
We have not been able to do more than make a rapid survey of the sense
of sight as it seems to be developed in the invertebrates and lower
animals. The visual organs differ, not only in structure, but in
principle. We may, I think, distinguish four types.
1. Organs for the mere appreciation of light or darkness (shadow),
exemplified by pigment-spots, with or without concentrating apparatus.
2. Organs for the appreciation of the direction of light or shadow, with
or without a lens. The simple retinal eyes of gasteropods, and perhaps
in some cases the ocelli of insects, probably belong to this class.
3. True eyes, or organs in which a retinal image is formed, through the
instrumentality of a lens, as in vertebrates and cephalopods.
4. The facetted eyes of insects, in which a stippled image is formed, on
the principle of mosaic vision.
Unfortunately, all these are called indiscriminately eyes, or organs of
vision. An infusorian or a snail is said to see. But the terms "eye,"
"vision," "sight," imply that final excellence to which only the higher
animals, each on its own line, have attained.
This final excellence probably has its basis and earliest inception in
the fact that the functional activity of protoplasm is heightened in the
presence of ætherial vibrations. If, then, we imagine, as a
starting-point, a primitive transparent organism with a general
susceptibility to the influence of light-vibrations, the formation
within its tissues of pigment-granules absorbent of light will render
the spots where they occur specially sensitive to the ætherial
vibrations. Special refraction-globules would also act as minute lenses,
focussing the light, and thus concentrating it upon certain spots.
[Illustration: Fig. 39.--Direction-retina.
Simple retina for distinguishing the direction of the source of light or
of shadow.]
In many of the lower animals we find such organs, belonging to our first
category, and constituting either eye-spots of pigmented material or
simple lenses covering a pigmented area. If we call these eyes, we must
remember that in all probability they have no power of what we call
vision--only a power of distinguishing light from dark. Where, however,
there exists beneath the lens a so-called retina, that is, a layer of
rod-like endings of a nerve, it might, at first sight, be thought that
there, at any rate, we have true vision. But in all probability, in a
great number of cases the retinal rods are simply for the purpose of
rendering the organism sensitive, not only to the presence of light, but
to its direction. Light straight ahead (_a_) stimulates the middle rods;
from one side (_b_, _c_) it is focussed on the rods of the opposite side
of the retina; and similarly for intermediate positions. The presence of
a retinal layer is thus no infallible sign of a power of vision as apart
from mere sensibility to light. Indeed, in a great number of cases, from
the convexity and position of the lens, the formation of an image is
impossible. Only when it can be shown that a more or less definite image
can be focussed on the retina, or can be formed on the principle of
mosaic vision, can we justly surmise that a power of true vision is
present. I doubt whether this can be shown to be unquestionably the case
in any forms but the higher arthropods, the cuttle-fishes and their
allies, and the vertebrates.
There is one more point for consideration before we leave the sense of
sight--Are the limits of vision the same in the lower forms of life as
they are in man? or, to put the question in a more satisfactory
form--Are the limits of sensibility to light-vibrations the same in them
as in us? M. Paul Bert concluded that they are. But Sir John Lubbock
has, I think, conclusively shown that they are not. For the full
evidence the reader is referred to his "Senses of Animals."[FM] His
experiments on ants, with which those of M. Forel are in complete
accordance, satisfied him that these little animals are sensitive to the
ultra-violet rays which lie beyond the range of our vision. Other
experiments with fresh-water fleas (_Daphnia_) showed that they have
colour-preferences, green and yellow being the favourite colours.
The daphnias were placed in a shallow wooden trough, divided by movable
partitions of glass into divisions. Over this was thrown a spectrum of
rainbow colours. The partitions were removed, and the daphnias allowed
to collect in the differently illuminated parts of the trough. The
partitions were then inserted, and the number of crustaceans in each
division counted. The following numbers resulted from five such
experiments:--
Dark. Violet. Blue. Green. Yellow. Red.
0 3 18 170 36 23
Special experiments seem to show that their limits of vision at the red
end of the spectrum coincide approximately with ours; but at the violet
end their spectrum is longer than ours. Sir John covered up the visible
spectrum, so as to render it dark, and gave the daphnias the option of
collecting in this dark space or in the ultra-violet. To human eyes both
were alike dark. But not so to the daphnian eye; for while only 14
collected in the covered part, 286 were found in the ultra-violet. The
width of the violet visible to man was two inches. Sir John divided the
ultra-violet into three spaces of two inches each. Of the 286 daphnias,
261 were in the space nearest the violet, 25 in the next space, and none
in the furthest of the three spaces. From which it would seem that,
though these little creatures are sensitive to light of higher
vibration-period than that which affects the human eye, their limits do
not very far exceed ours. We have seen that human beings differ not a
little in their limits of violet-susceptibility. We may presume that Sir
John Lubbock and those who assisted him in these experiments were normal
in this respect. But it is possible that some individuals could have
perceived a faint purple where there was darkness to them, and that the
majority of the 261 daphnias were collected in the region just beyond
the partition between ultra-violet and darkened violet. Still, there is
no cause for doubting the general conclusion that daphnias are sensible
to ultra-violet rays beyond the limits of human vision.
* * * * *
[Illustration: Fig. 40.--Antennary structures of hymenoptera. (After
Lubbock.)
a., cuticle; b., hypodermis; c., ordinary hair; d., tactile hair; e.,
cone; f., depressed hair lying over g. cup with rudimentary hair at the
base; h., simple cup; i., champagne-cork-like organ of Forel; k.,
flask-like organ; l., papilla, with a rudimentary hair at the apex.]
Sir John Lubbock has an interesting chapter on problematical organs of
sense. In the antennæ of ants and bees there are modified hairs and pits
in the integument (at least eight different types, according to Sir John
Lubbock), the sensory nature of which is undoubted. But what the sensory
nature in each case may be is more or less problematical. Many worms
have sense-hairs or bristles of the use of which we are ignorant. Some
organs described as tactile or olfactory in the lower invertebrates are
so described on a somewhat slender basis of evidence. The sense-value of
the bright marginal beads of sea-anemones is unknown. Even in animals as
high in the scale of life as fishes, there is a complete set of
sense-organs--the muciparous canals, in the head and along the lateral
line down the side, the function of which we can only guess. By some
they are regarded as olfactory; by others, as fitted to respond to
vibrations or shocks of greater wave-length than the auditory organ can
appreciate; by others, as of importance for the equilibration or
balancing of the fish.
It will thus be seen that, apart from the possibility of unknown
receptive organs as completely hidden from anatomical and microscopic
scrutiny as the end-organs of our temperature-sense, there are in the
lower animals organs which may be fitted to receive modes of influence
to which we human folk are not attuned.
And what are the physical possibilities? We have seen that, through the
telæsthetic senses--hearing, vision, and the temperature-sense--we are
made aware of the vibrations of distant bodies, the effects of which are
borne to us on waves of air or of æther. The limits of hearing with us
are between thirty and about forty thousand (or perhaps, in very rare
cases, fifty thousand) vibrations per second. But these are by no means
the limits of vibrations of the same class. By experiments with
sensitive flames,[FN] Lord Rayleigh has detected vibrations of fifty-six
thousand per second; and Mr. W. F. Barrett has shown that a sensitive
flame two feet long is sensitive to vibrations beyond the limit of his
own hearing and that of several of his friends who were present at the
experiment. We have some reason to suppose that vibrations too rapid to
be audible by man are audible by insects, but not much is known with
regard to the exact limits.
The following table shows what is known concerning the æther-vibrations.
The figures are those given by Professor Langley:--
Wave-lengths Number of
in thousandths vibrations
Quality of radiations. of a per second Effects on man.
millimetre. in billions.
Limit of photography,
artificial source 0.185 160 none known
Limit of photography,
solar source 0.295 none known
Limit of violet to normal eyes 0.36 833 }
Limit of red to normal eyes 0.81 370 } vision.
Probable inferior limit of
temperature-sensations 9.25[FO] 30 temperature-sense
Longest waves hitherto
recognised with bolometer 30.0 1 none known
From this table it will be seen that, apart from the possible extension
of sight beyond human limits, there are possibilities of another sense
for the ultra-violet actinic vibrations as different from sight as is
the infra-red temperature-sense. Moreover, the temperature-sense for us
has no scale; there is nothing corresponding to pitch in sound or colour
in sight. It may not be so with lower organisms. Insects, for example,
may be sensitive to tones of heat. The bee may enjoy a symphony of solar
radiance. I am not saying that it is so; I am merely suggesting
possibilities which we have not sufficient knowledge to authoritatively
deny. We have no right to impose the limits of human sensation on the
entire organic world. Insects may have "permanent possibilities of
sensation" denied to us.
Even within our limits there may be, as we have already seen, great and
inconceivable differences. We saw that our own colour-sensations are
probably due to the blending and overlapping in different proportions of
three primitive monochromatic bands, but that in all probability in
birds the bands are different, and overlapping is largely prevented.
Their colour-phenomena must be inconceivably different from ours. And
what shall we say of the colour-vision of invertebrates? Are we
justified in supposing that for them, as for us, R., G., and V. are the
unstable explosives, and that they are present in the same proportions
as with us? If not, their colour-world cannot be the same as ours. Of
the same order it probably is. And all that we can hope to do is to
show, as has been shown, that colours which differently affect us affect
them also differently.
* * * * *
In conclusion, we may return to the point from which we set out. The
organism is fitted to respond to certain influences of the external
world. The organs for the reception of these influences are the
sense-organs. When they are stimulated waves of change are transmitted
inwards to the great nerve-centres; they are there co-ordinated, and
issue thence to muscles or glands. Thus the organism is fitted to
respond to the influences from without. The activities of organisms are
in response to stimulation.
We have seen that the cells of the organic tissues are like little
packets of explosives, and that the changes which occur in the organism
may be likened to their explosion and the setting free of the energy
stored up in them. The end-organs of the special senses may be regarded
as charged with explosives of extreme sensitiveness. Some are fired by a
touch; the molecular vibrations of sapid or odorous particles explode
others; yet others are fired by the coarser vibrations of sound; others,
once more, by the energy of the ætherial waves. The visual purple is a
highly unstable chemical compound of this kind; expose it for a moment
to light, and it topples over to a new molecular arrangement, the colour
being at the same time discharged. If the retina has been removed from
the body, this is all that happens. But if (in the frog) it be replaced
on the choroid layer from which it has been stripped, the visual purple
is reformed. The explosive is thus reconstructed and the sensibility is
restored. Thus, as fast as the explosives are fired off by
sense-stimuli, so fast in normal life are they reconstituted and the
sensibility restored. Meanwhile the explosion at the end-organs has
fired the train of explosives in the nerve, and created molecular
explosive disturbances in the brain. Thence the explosive waves pass
down other nerves to muscles or glands, and, giving rise therein to
further explosions, take effect in the activities of the organism.
We shall have to consider these activities hereafter. We must now turn
to the psychical or mental accompaniments of the explosive disturbances
in the brain or other aggregated mass of nerve-cells.
NOTES
[EO] See abstract in _Nature_, vol. xxxiv. p. 515.
[EP] See _Nature_, vol. xxxvii. p. 557.
[EQ] "Sense-Organs and Perception of Fishes:" Journal of Marine
Biological Association, New Series, vol. i. No. 3, p. 225.
[ER] _Nature_, vol. xlii. p. 201.
[ES] _Nature_, vol. xxxvi. p. 273.
[ET] Journal of Marine Biological Association, New Series, vol. i. No.
3, p. 235.
[EU] Mr. S. Klein mentions a similar fact in connection with _Bombyx
quercus_ (_Nature_, vol. xxxv. p. 282).
[EV] Journal of Marine Biological Association, New Series, vol. i. No.
2, p. 211.
[EW] A friend of mine informs me that his limit is about 17,500 per
second, 20,000 being quite inaudible.
[EX] Journal of Marine Biological Association, New Series, vol. i. No.
3, p. 251.
[EY] Of course, anglers will say that what may be true for pollack and
other coarse and vulgar sea-fish does not apply to King Salmon or
Prince Trout.
[EZ] "Senses of Animals," p. 117.
[FA] See a very interesting and lucid paper by Professor Crum Brown,
whose name is intimately connected with this subject, in _Nature_,
vol. xl. p. 449.
[FB] It is interesting to note that in the blind-fish (_Amblyopsis
spelæus_) the semicircular canals are, according to Wyman,
unusually large.
[FC] The dampers must, of course, be lifted by depressing the loud
pedal.
[FD] "Special Physiology," p. 636.
[FE] A band and not a line, because R. is unstable to the impact of a
considerable range of light-vibrations.
[FF] Mr. Chattock has kindly supplied me with the following note:--
"Readings at the violet end were taken at the extremity of the
lavender rays, at the point where the faint band of lavender light
seemed to end off about half-way across the field of view (the
cross-wires being invisible).
"At the red end the cross-wires were always visible, and were in
each case set to the point where the top horizontal edge of the
spectrum lost its definition.
"Other things equal, the 'red' readings should be more reliable than
the violet, therefore, from the greater definiteness of the point
observed, and the means of observing it. But against this has to
be set off the fact that the extreme violet rays were spread out
by the prism used more than eight times as much as the red rays.
"In any case, the wide differences observed in the 'red' readings
are much greater than could have been due to misunderstanding or
careless observation--as shown by setting the instrument to
maximum and minimum readings, and noting the very obvious
difference between them apparent to a normal eye. The same
conclusion is rather borne out by the closer (average) agreement
between the two eyes of the same individual than between those of
different persons.
"The source of light was the central portion of an ordinary Argand
burner."
[FG] The variations above indicated throw light on a fact to which Lord
Rayleigh has directed attention. The yellow of the spectrum may be
matched by a blending of spectral red and spectral green; but the
proportions in which these spectral colours must be mixed differ
for different individuals. The complementary colours for different
individuals are also not precisely the same.
[FH] "Colour-Vision and Colour-Blindness," R. Brudenell Carter
(_Nature_, vol. xlii. p. 56).
[FI] Journal of Marine Biological Association, New Series, vol. i. Nos.
2 and 3. His experiments with regard to the colour-sense in fishes
gave, for the most part, negative results.
[FJ] We must remember how largely the antennæ are used when an insect
is finding its way about. Watch, for example, a wasp as it climbs
over your plate. If the antennæ be removed, it seems to stumble
about blindly. The antennæ seem almost to take the place of eyes
at close quarters.
[FK] "Senses of Animals," p. 194.
[FL] See _Nature_, vol. xli. p. 407.
[FM] Chap. x. p. 202.
[FN] The observations are not yet published, and I have to thank Lord
Rayleigh for his courtesy in allowing me to make use of this fact.
[FO] Professor Langley finds that the maximum effect with a radiating
source
at 170° C. is at about 5.0 thousandths of a millimetre wave-length.
" 100° C. " " 7.5 " " "
" 0° C. " " 11.0 " " "
We are sensitive to radiations from a body at 100° C. But when the
temperature falls below the normal temperature of the body we are
not sensitive to heat-vibrations, but to loss of heat from the
surface exposed. The limit of sensibility to heat-vibrations,
therefore, probably lies between 7.5 and 11 thousandths of a
millimetre. I have taken about 9.25 as the limit.