external world reaches the central nervous system. Sensations of touch,
temperature, and pain are fairly easy to understand, since the nerves
which convey such impressions have numerous endings in the skin. End
organs of nerves in the joints and muscles doubtless enable the animal to
perceive and estimate strain and resistance in moving or lifting things.
But the power of perceiving the chemical peculiarities of things; light,
involving the formation of visual images, which we call seeing; sound;
and position and equilibrium, it is not possible for the whole surface
of the body to possess. The principle of division of labour is extended
to the task of perception as well as to that of motion; and cells, with
their property of responding to light, vibration, chemical stimulation,
etc., are grouped together to form special organs, connected with the
central nervous system by special nerves.
Perhaps the most important factor which can influence protoplasm is the
chemical nature of its surroundings; and in the first essay, on the
general nature of protoplasm, we touched upon the way in which it is
drawn towards some substances, and repelled by others.
In the body there are two sets of cells deputed to act for the rest in
this particular. One set is situated in the membrane lining the nose,
over which the air we breathe passes; and these cells examine our
gaseous surroundings, and warn us, by what we term ‘smell,’ whether the
atmosphere is fit for us or we had better seek a purer. The other set
is for the examination of liquids. Against these we are protected by
our skin, and, as we do not absorb anything through it, it is devoid of
the power of examining the things it touches. But with our food it is
different; we must have the power of testing that. Accordingly, there
are Customs officers in our mouth in the form of little groups of cells,
which report upon the liquids and solids moistened by saliva, and enable
the animal to reject pernicious imports. Thus, the stimulation of a small
portion of the protoplasm composing a body is transmitted over the whole,
and is able to awake in it the necessary response.
[Illustration: DIAGRAM 51.]
[Illustration: DIAGRAM 52.]
[Illustration: DIAGRAM 53.]
[Illustration: DIAGRAM 54.]
[Illustration: DIAGRAM 55.]
[Illustration: DIAGRAM 56.]
So much for the chemical sense organs; they are comparatively simple.
But between a single cell, which always makes towards or always hurries
out of a ray of light passing through the water in which it swims, and
an animal with eyes capable of recognising the colour, shape, size, and
distance of objects in space, there really does seem to be a wide gulf.
It is not, however, too wide to be bridged.
After the single-cell stage has been passed, and we have beasts
consisting of an inner layer of cells which is digestive in function,
and an outer layer which is protective, motor, and sensory, the power
of perceiving light is doubtless vested in the outer layer. When we get
beasts consisting of three layers progressing along the straight path
of development which leads to man, we find the outer layer becoming
too opaque for this purpose, and the torch is handed on to the sensory
tube derived from it. (See Diagram 5.) As more and more protection is
required, the skin thickens, and the neural tube comes to lie deeper,
as in Diagram 51. In order not to lose the light altogether, it has to
throw out buds, which concentrate in themselves the peculiar faculty of
perceiving it, and at the same time little pits are formed in the skin
just over them to help the light to reach them. (See Diagram 52.) In
Diagram 53 both the nervous elements and the integumentary are developing
their possibilities; and in Diagram 54 a large surface has been prepared
for the reception of light, and a lens formed to focus the rays upon it.
Diagrams 55 and 56 give the concluding stages in the development of the
eye: the formation of the cornea and its protecting eyelids. The two
cavities are filled with clear liquids, and the whole eyeball supported
by connective tissue.
So fascinating is everything connected with the eye that the temptation
to describe it in detail is great; but in a book of rough outlines,
and in consideration of the many important matters yet awaiting their
turn, we must confine ourselves to briefly mentioning a few of the more
important points concerning it. The light is focussed by the lens upon
the nervous curtain at the back, and produces there a picture, as in
the photographic camera. Thus we perceive the shape of objects. The
different rays of the spectrum affect different elements in this curtain
or retina, whereby we get sensations of colour. Finally, the clearness
of the picture, its size, the degree of convergence of the two eyes, and
the effort of focussing—for the curvature of the surface of the lens
can be altered—enable us to estimate the size and distance of an object.
And now, though it would take volumes to do justice to the physiology of
vision, we must pass on to deal equally briefly with the functions of
that no less important organ, the ear.
The essential part of the ear is a membranous bag, formed by the pouching
in of the outer layer of cells—as shown in Figs. 1, 2, and 3 of Diagram
57—which comes to lie in a bony chamber beneath the skull, and assumes
the somewhat complicated shape depicted in Fig. 4. We have not time,
nor is it for our purpose necessary, to trace all the steps in the
development of the ear, either external or internal, nor need we spend
much time upon its structure, beyond indicating its position. But its
position, which is shown in Diagram 58, must be grasped in order to
understand how it is influenced by sound.
[Illustration: DIAGRAM 57.—SHOWING DEVELOPMENT OF THE MEMBRANOUS
LABYRINTH OF THE EAR.
U, Utricle; C, cochlea; S, saccule; S.C., semicircular canals.]
It will be seen that the membranous bag, which is fitly termed the
labyrinth, is situated in a bony cavity which fits so closely as to be
termed the bony labyrinth (C). The membranous labyrinth is filled with
a liquid, called endolymph, and the bony labyrinth (C) is also filled
with a liquid, called perilymph, in which the membranous bag swims. All
this is called the inner ear. The inner ear communicates with a second
cavity—the middle ear (B)—by two apertures in the bony wall, which are
closed by membranes. The middle ear is full, not of liquid, but of air,
and is separated from the external ear, the cavity marked A, which is
open to the external world, by another membrane called the tympanum, or
drum, of the ear. The middle ear is connected by a tube with the throat,
so that the pressure of the air on both sides of the drum may be the same.
[Illustration: DIAGRAM 58.—SHOWING THE POSITION OF THE EAR.
A., Outer ear; B., middle ear; C., inner ear.]
Now, the object of this arrangement is that the ear may be able to fulfil
one of its principal duties, namely, the perception of sound. Sound, as
the reader is doubtless aware, is transmitted through the air as waves of
condensation and rarefaction, due to the swinging backwards and forwards
of its particles; it resembles the passing on of a bump along a line
of trucks on the railway when the engine runs up against the end one
preparatory to coupling. The magnitude of this oscillation we perceive as
the loudness, the frequency as the pitch of a note. Now, when the waves
of sound strike against the drum of the ear, they cause it to vibrate
backwards and forwards also. Supposing there was no middle ear, and the
sound waves beat directly upon the membranous windows of the inner ear,
these could not be made to vibrate, as there is liquid behind them, and
liquids are incompressible; so, in order that the movements of the drum
may be transmitted to the liquids of the inner ear, they are carried
across the middle ear by a chain of small bones, by which their extent
is curtailed, but their force increased, and brought to bear upon one
only of the two openings. The consequence of this is that the membrane
closing it is able to vibrate and pass on the vibrations to the liquid
within, since when it is pushed in, the membrane covering the other hole
is pushed out.
[Illustration: DIAGRAM 59.—THE SEMICIRCULAR CANALS.]
Exactly how the different parts of the membranous labyrinth contribute
to our perception of sound we do not quite know. It appears as though
the difference of pressure in saccule and utricle originally conveyed to
the brain a sensation of noise without any idea of quality, while the
cochlea was developed later to analyze sounds and give information as to
pitch and tone. Whether the rest of the labyrinth has any longer a part
to play in the perception of sound, we cannot say with certainty; but it
seems pretty certain that the cochlea is the organ for receiving musical
impressions. Here, again, though, we are at a loss, for we do not know
with certainty how the cochlea acts. In shape it is a long tube, and in
the head is coiled spirally—like a snail’s shell to look at. Along its
whole length is a ridge of cells with short hairs projecting from their
inner surface into the liquid it contains; and to the cells along this
ridge a branch of the auditory nerve is distributed. But as to whether
one of the cells along this keyboard responds to each of the notes we
can distinguish, or whether they are affected as a whole, physiologists
are not yet agreed.
At least one other important duty the ear performs; it tells us in what
position we are, and how our whole head moves or is moved. On the top of
the saccule, in Diagram 57, Fig. 4, there are shown three little loops
which are called the semicircular canals. They are shown again more
clearly by themselves in Diagram 59.
Fig. 1 shows their position with regard to each other. It will be seen
that two of them are vertical, with their loops forming a right angle
with one another, and that the other is horizontal—in fact, that they lie
in the three planes of space. Fig. 2 shows the structure of one of them;
it has a swelling at one end (_a_), and a knob projecting into it where
the nerve joins it (_b_). In Fig. 3 is shown a section through this knob,
which gives the key to the use of these structures. A little head of
cells projects from the wall of the canal into its lumen, and from these
cells hairs bristle out into a dome-like covering of jelly, weighted, to
prevent its moving too easily, with small particles of lime. Now, if you
take up a round vessel full of liquid—say a bowl of gold-fish—and give
it a twist round, you will notice that, though the bowl turns, the water
inside does not; the fish remain in their old position. If there were
a rod projecting from the side of the bowl, it would, of course, move
with it, and if a fish came in its way would strike against it. This is
the principle of the semicircular canal. For if we turn our head, the
tube of the canal turns, passing over the liquid in it, which of course
does not move, though it appears to flow in the opposite direction. The
consequence is that the hairs on the side of the knob in the direction
in which the head is being moved are pressed upon by the dome of jelly,
which, as it floats in the liquid, tends to remain where it is. The
nerves, stimulated in this way, inform the animal generally of the
movement.
These little organs are very important to us, though we have our eyes to
correct our ideas of position, and they are still more so to the fish,
which dart and turn in the wide expanse of the ocean, and the birds and
bats, which wheel about in the air. There are, however, some occasions
when we do not feel inclined to bless them; for, inasmuch as they
faithfully report every roll and plunge of a ship to a person on board,
it is they which are mainly responsible for sea-sickness.
* * * * *
And now that we have seen how the body lies with regard to the external
world; how it is efficiently protected from its surroundings; how it
is placed in communication with them; and have briefly examined the
organs by which it makes its chemical and physical investigations,
looks out into space, and is kept aware of what is going on therein, we
may return to the means whereby it responds as a whole to the stimuli
thus reported—the central nervous system—and try to learn how the right
response is brought about.
IV.
There is but one thing more to describe in the mechanism of the body—the
connecting link between the last two sections. In the last we saw how the
body receives stimuli from the external world; in the one before, that
when these stimuli reach the central nervous canal it in turn stimulates
the organs to perform such movements as circumstances require. What,
therefore, remains to be described is the working of that canal by which
these necessary movements are ordered and controlled.
Now, in speaking of reflex action a few pages back, we said that the
nerves which bring in stimuli from the periphery distribute them about
the neural canal to those cells whose activity, by sending out fresh
stimuli to the muscles, produces the requisite movements. These motor
cells, however, are not scattered about the spinal cord anyhow. They are
collected into clusters, or nuclei, as they are sometimes called, and
each cluster has special duties—_i.e._, a special organ to control. Thus,
we say that there are in the central nervous system centres—a nervous
centre to control the leg; another to work the diaphragm; another
for the muscles of the ribs; more for the arm, hand, etc. And these
centres are in communication with one another, so that they may not pull
different ways.
In the first example of reflex action given in Section II. of this
essay, the sensation of a pin-prick was first conveyed to the centres
controlling the limb injured, by whose activity it was drawn away from
the danger. But the nerve which gave the warning which produced this
elementary movement distributed the impression that something was wrong
to the higher centres, so that the whole body was involved in protecting,
doctoring, and avenging the outraged member; from which it would appear
that the lower centres are under control of higher ones. And this is the
case. If we may be allowed the metaphor, there are captains of tens,
who are under the direction of captains of fifties, and the captains of
fifties receive their orders from captains of hundreds. The nerve canal,
the manner of whose formation as a simple tube is shown in Diagrams 5 and
42, has therefore different functions in different parts, and this to
such an extent that considerable differentiation in bulk and structure is
produced.
The neural canal may be roughly divided into two parts—a comparatively
simple tube, running the greater part of the animal’s length, containing
many centres from which nerves run to the organs they control; and a
complicated bulbous enlargement at one end, with thickened walls, in
which are the centres controlling those in the cord, and thereby managing
not so much organs as the whole animal. The former is called the spinal
cord, the latter the brain.
This division, accustomed as we all are to take it for granted, offers
plenty of food for reflection. Why should an animal have such a brain
placed in its head? Why, indeed, should it have a head, regarding that
member as a group composed of eyes, nose, mouth, ears and brain? The
mouth gives us the key to the riddle; the mouth is the essential organ,
and all the rest are its accessories.
In the first essay we saw that the basis of life was chemical, and in the
second that the materials necessary for the chemical action, or food,
must, in the higher animals, be taken into the digestive tube through the
opening which we call the mouth. Therefore, as it is highly important
that only the most beneficial substances shall be received into it, and
that all which are actively injurious shall be excluded, it is plain that
the organs of chemical perception must be placed in its neighbourhood—the
organs of smell to enable the mouth to find its food, and the organs
of taste to aid in selecting it. As, moreover, our humble ancestors,
the fishes, move literally mouth foremost, it is not surprising to
find the organs of space perception, the eyes, also situated in its
neighbourhood, especially when one considers that their food is often
of a lively character, and requires precision of movement to secure it.
The inevitable consequence of thus grouping the more important organs of
perception under the fore-end of the neural canal is that it grows and
develops more highly here than elsewhere along its length, and soon is
in a position to dictate to the rest of the body. Another reason why it
must develop is that it must contain centres for turning its impressions
to practical account, not only by producing complicated movements in the
jaws, eyes and gills, but also by ruling the centres in the cord, and
instructing the body to carry the mouth whither it needs to go.
[Illustration: DIAGRAM 60.—SHOWING PRIMARY DIVISION OF NERVOUS TUBE.]
In the preceding diagram (60) the origin of the brain is shown as a
dilatation of the end of the neural canal into a bulb with thickened
walls, which has already become constricted in places, so that it is
subdivided into three. The next diagram (61) is intended to give, in
no matter how crude and schematic a way, some idea of the lines on
which the development continues. We do not show all, or even half, the
structures which go to make up the brain. To do so would be out of place
in a book like this. Further, we shall endeavour as far as possible to
speak of the brain in general terms, avoiding the five-syllable bastard
Græco-Latin names with which the early anatomists have endowed almost
every square inch of its substance, and confine ourselves to summing up
its functions as briefly as can be done with justice.
[Illustration: DIAGRAM 61.—GIVING A ROUGH IDEA OF HOW THE BRAIN IS
DEVELOPED.]
In pursuance of this method, attention must be drawn to the fact that
only the foremost of the three original bulbs (marked A in the diagram)
and the hindermost (C) continue to grow. The middle one (B) remains
comparatively simple. From the foremost lobe buds grow out to form the
eyes in the manner which we have already described, and other buds push
forwards to meet the nerves from the nose. The latter have, even in the
early stages shown in Diagram 61, reached an extraordinary size; and
when we come to trace them further, we shall find that they become very
complex, and acquire remarkable and unexpected powers, considering their
humble origin. Strange changes also take place in the hindermost bulb.
It splits along the top, so that the cavity it contains is open like a
saucer, though bridged over by a three-lobed body called the cerebellum.
Following the spinal cord up into the brain, we are conscious of no
sudden line of demarcation separating the one from the other, only of an
increasing size and complexity. The lower parts of the brain send out and
receive nerves much as the cord does; three pairs go to the muscles which
turn the eyes; other pairs bring in sensations from the face and throat;
others control the muscles of the face, tongue and throat. But the brain
differs from the cord in being directly connected by nerves, not only
with adjacent parts, but also with the distant and more important organs
in the interior of the body—heart, lungs, etc.; in containing groups of
cells which have stimuli sent on to them from all over the body viâ the
cord; and in possessing centres which control those lower down in the
nervous system. It therefore not only receives and balances stimuli from
all over the body, but, by governing the centres which preside over the
bodily movements, is able to wield and direct the body as a whole.
The hinder divisions of the brain, which we shall consider first, have
no connection with consciousness or volition. They only produce reflex
movements, which, however, owing to the wealth of material they have to
work upon, are wonderfully complex and far-reaching.
Let us take a few examples. In the hindermost division of the brain (C
in the diagrams) there is the centre which presides over the oxygen
supply, the importance of which we saw in the essay on vital chemistry.
This centre perceives when the lungs have been filled with a gas, and
causes them to be emptied; it perceives when they are empty, and again
does not allow them to remain too long in that state, before ordering an
inspiration; it notes the quality of the air which is passing through the
nose, and it notes the quality of the blood which bathes its own cells.
The condition of the blood, indeed, is closely watched. An excessive
quantity of carbonic acid gas, poverty of oxygen, even temperature, all
produce through it an effect upon the rhythm of the breathing.
Close by the respiratory centre is the centre which controls the
circulation. But enough has been said in the section on reflex action,
wherein the process of fainting was described, to give an idea of the
part it plays in the body; so it need not detain us here.
We cannot, however, pass over its neighbour, the centre of temperature,
so briefly. Its methods not only afford one of the most striking and
interesting examples of harmonious regulations by reflex action, but the
subject of temperature itself is so important that we must describe in
some detail how that of the body is kept level.
As we said when discussing protoplasm generally, life—that is, the
change always going on in the protoplasmic substance—is influenced by
temperature: the single cell becomes less active at a low temperature,
and dies at a high one; so obviously there is a temperature at which its
functions are most easily carried on. Inside the body the cells are all
kept at the temperature best for them by the circulation of the blood;
but the absolute temperature of the whole body depends upon the heat
which is generated within it by chemical action, and the heat which it
loses to, or receives from, its surroundings. Under normal conditions
this temperature in man is 98·4° F., when the production of heat from
its own metabolism is balanced by the loss of heat by radiation. If,
however, the atmosphere be very hot, less heat is developed in the body,
the general metabolism being slower; and more heat is lost, since by
reflex action the skin is bathed in sweat and cooled by its evaporation,
and the small bloodvessels under the skin are dilated, so that more
blood being brought to the surface, its chance of being cooled by
radiation is thereby increased. If, on the other hand, the atmosphere
is cool, the loss at the surface is minimized by constriction of the
cutaneous bloodvessels, and a checking of the perspiration and consequent
evaporation; while internally more heat is generated by increased
metabolism. The cells which are mainly responsible for the production
of heat are those of the muscles; and when much heat is required they
increase in activity, not only in their general tone, but even by a
visible movement, which we describe as shivering. So, within reasonable
limits, whatever the temperature of its surroundings may be, that of the
body remains the same, and though we may raise or lower our temperature
by lying in a hot or cold bath, reflex adjustment of the sweat glands,
bloodvessels and muscles brings it quickly back to normal when we emerge.
With a passing mention of the cerebellum, the three-lobed organ shown in
Diagram 61, and seen again in a more advanced stage in Diagram 63, we may
dismiss the two hinder divisions of the brain.
The cerebellum lies on the upward path of fibres from the cord to the
higher centres in the fore-brain. It is a somewhat complicated organ,
and its functions are not yet fully known. The older physiologists took
a very extreme view of its importance, assigning to it, among other
romantic duties, that of providing a habitation for the soul. This
opinion on the strength of later research we can hardly endorse. The
cerebellum really seems mainly concerned in co-ordinating the action
of the muscles, especially in maintaining equilibrium in standing and
walking.
Our knowledge of the whole brain is very far from complete. We should
like to know the peculiar function of each little group of cells that
can be made out under the microscope, and the paths of all the fibres
connecting the different parts of the nervous system. As it is, we have
to wait with the best patience we can while they are being investigated,
and hope. In few departments, however, have the labours of the
physiologist proved more fruitful and interesting than in the study of
the fore-brain (A in the diagrams).