conspicuous. Their use is, to give firmness and shape to the body.
Some of them likewise serve as armour, or defense, to guard
important parts; thus the skull is admirably contrived to defend
the brain; and the spine or backbone is designed, not only to
strengthen the body, but to shield that continuation of the brain,
called the spinal marrow, from whence originate great numbers of
nerves, which pass through convenient openings of this bone, and
are distributed to various parts of the body. In the structure of
this, as well as every other part, the wisdom of the Creator is
manifest. Had it been a single bone, the loins must have been
inflexible; to avoid which, it consists of a number of small
bones, articulated or joined together with great exactness, which
are strengthened by compact ligaments. Hence it becomes capable of
various inflections, without injuring the nerves, or diminishing
that strength which is so much required.
The whole system of bones, or skeleton, is constructed of several
parts, of different shapes and sizes, joining with one another in
various manners, and so knit together, as best to answer to the
motions which the occasions of the animal may require.
These bones serve as levers for the muscles to act on; which last
serve as mechanical powers, to give the machine various motions,
at the command of the will.
The muscles are fleshy fibres, attached by their extremities to
the bones. When the fibres shorten themselves, the two parts into
which the muscle is inserted are brought nearer; and, by this
simple contrivance, all the motions of animals are performed, and
their bodies carried from one place to another.
Joints are provided with muscles for performing the motions for
which they are adapted; every muscle pulling the bone, to which it
is attached, in its own particular direction. Hence the muscles
may be considered as so many moving forces, as was before hinted;
and their strength, the distance of their insertion from the
centre of motion, the length of the lever to which they are
attached, and the weight connected with it, determine the duration
and velocity of the motions which they produce. Upon these
different circumstances depend the different kinds of motion
performed by various animals, such as the force of their leap, the
extent of their flight, the rapidity of their course, and their
address in catching their prey.
Most of the muscles act upon the bones, so as to produce the
effects of a lever of the third kind, as it is termed by
mechanics, where the power acts between the centre of motion and
the weight; hence it has a mechanical disadvantage; as an instance
of this, the muscle which bends the forearm, is inserted about one
eighth or one tenth of the distance from the centre of motion that
the hand is, where the weight or resistance is applied; hence the
muscle must exert a force eight or ten times greater than the
weight to be raised. But this disadvantage is amply compensated by
making the limbs move with greater velocity; besides, if room had
been given for the muscles to act with greater advantage, the
limbs must have been exceedingly deformed and unwieldy. [1]
The muscles, in general, at least those which serve for voluntary
motion, are balanced by antagonists, by means of which they are
kept beyond their natural stretch. When one of two antagonists is
contracted by the will, the other relaxes in order to give it
play; or at least becomes overpowered by the contraction of the
first. Also when one of such muscles happens to be paralytic, the
other being no longer balanced, or kept on the stretch,
immediately contracts to its natural length, and remains in that
situation. The part to which it is fixed will, of course, be
affected accordingly. If one of the muscles which move the mouth
sideways be destroyed, the other immediately contracting, draws
the mouth awry; and in that situation it remains. The same may be
observed of the leg, the arm, and other parts. Some muscles assist
one another in their action, while others have different actions;
according to their shapes, the course of their fibres, and the
structure of the parts they move.
According to the shape and nature of the bones to be moved, and of
the motions to be performed, the muscles are either long, or
short; slender, or bulky; straight, or round. Where a great motion
is required, as in the leg, or arm, the muscles are long; where a
small motion is necessary, they are short; for a strong motion
they are thick, and for a weak one slender.
Some of the muscles are fastened to, and move bones; others
cartilages, and others again other muscles, as may best suit the
intention to be answered.
With respect to the bones, some are solid and flattened; others
hollow and cylindrical. Every cylindrical bone is hollow, or has a
cavity containing a great number of cells, filled with an oily
marrow. Each of these cells is lined with a fine membrane, which
forms the marrow. On this membrane, the blood vessels are spread,
which enter the bones obliquely, and generally near their middle;
from some branches of these vessels the marrow is secreted; while
others enter the internal substance of the bones for their
nourishment; and the reason why they enter the bones obliquely is,
that they may not weaken them by dividing too many fibres in the
same place.
The bones being made hollow, their strength is greatly increased
without any addition to their weight; for if they had been formed
of the same quantity of matter without any cavities, they would
have been much weaker; their strength to resist breaking
transversely being proportionate to their diameters, as is evident
from mechanics.
All the bones, excepting so much of the teeth as are out of the
sockets, and those parts of other bones which are covered with
cartilages, are surrounded by a fine membrane, which on the skull
is called pericranium, but in other parts periosteum. This
membrane serves for the muscles to slide easily upon, and to
hinder them from being lacerated by the hardness and roughness of
the bones.
But though the apparatus which I have been describing is admirably
contrived for the performance of motion; it would continue for
ever inactive, if not animated by the nervous system.
The brain is the seat of the intelligent principle: from this
organ, white, soft, and medullary threads, called nerves, are sent
off to different parts of the body: some of them proceed
immediately from the brain to their destined places, while the
greater number, united together, perforate the skull, and enter
the cavity of the backbone, forming what we call the spinal
marrow, which may be regarded as a continuation of the brain.
Portions of the spinal marrow pass through different apertures to
all parts of the body.
We are not conscious of the impression of external objects on our
body, unless there be a free communication of nerves, between the
place where the impression is made and the brain. If a nerve be
divided, or have a ligature put round it, sensation is intercepted.
There is perhaps only one sense which is common to all classes of
animals, and which exists over every part of the surface of the
body; I mean the sense of touch. The seat of this sense is in the
extremities of the nerves distributed over the skin; and by means
of it we ascertain the resistance of bodies, their figure, and
their temperature.
The other senses have been thought to be only more refined
modifications of the sense of touch; and the organs of each are
placed near the brain on the external surface of the head. The
sense of sight, for instance, is seated in the eye; the hearing in
the ear; the smell in the internal membrane of the nose; and the
taste in the tongue.
The light; the pulses, or vibrations of the air; the effluvia
floating in the atmosphere; saline particles, or particles which
are soluble in water or saliva, are the substances which act upon
these four senses; and the organs which transmit their action to
the nerves, are admirably adapted to the respective nature of
each. The eye presents to the light a succession of transparent
lenses to refract its rays; the ear opposes to the air membranes,
fluids, and bones, well fitted to transmit its vibrations; the
nostrils, while they afford a passage to the air in its way to the
lungs, intercept any odorous particles which it contains, and the
tongue is provided with spongy papillae to imbibe the sapid
liquors which are the objects of taste.
It is by these organs that we become acquainted with what passes
around us; by these we know that a material world exists. We may
however observe, that the nervous system, besides making us
acquainted with external things, gives us notice of many changes
that take place within our own body. Internal pain warns us of the
presence of disease; and the disagreeable sensations of hunger,
thirst, and fatigue, are signs of the body standing in need of
refreshment or repose.
Concerning the manner in which we become acquainted with external
things, by means of the senses, we know nothing. Many hypotheses
have been offered to explain this: none of them however are the
result of experiment and observation. Many philosophers have
supposed the universe to be filled with an extremely subtile
fluid, which they have termed ethereal; and this hypothesis has
been sanctioned by the illustrious authority of Newton. He however
merely offered it in the modest form of a query, for the attention
of other philosophers; little thinking that it would be made use
of to explain phenomena which they did not understand. His query
about a subtile elastic fluid pervading the universe, and giving
motion and activity to inert masses of matter, and thereby causing
the phenomena of attraction, gravitation, and many other
appearances in nature, was immediately laid hold of by his
followers, as a fact sufficiently supported, because it seemed to
have the sanction of so great an authority.
This hypothesis was made use of to explain a great number of
phenomena, and the physiologists, whose theories were generally
influenced by the prevailing philosophy, eagerly laid hold of it
to explain the phenomena of sensation, and muscular motion. When
an impression was made upon any part of the external surface of
the body, whether it was occasioned by heat, or mechanical
impulse, they supposed, that the ether in the extremities of the
nerves was set in motion. This motion, from the energy of the
ether, is communicated along the nerves to the brain, and there
produces such a change as occasions a consciousness of the
original impression, and a reference in the mind to the place
where it was made. Next they supposed, that the action of the will
caused a motion of the ether to be instantly propagated along the
nerves that terminate in the fibres of the muscles, which
stimulated them to contraction.
Other philosophers imagined, that a tremulous motion was excited
in the nerves themselves, by the action of external impulses, like
the motions excited in the string of a harp. These motions they
supposed to be propagated along the nerves of sense, to the brain,
and from thence along the motory nerves, to the muscles.
Before they attempted this explanation of the phenomena, they
should have proved the existence of such a fluid, or at least
brought forward such circumstances, as rendered its existence
credible. But supposing we grant them the hypothesis, it will, in
my opinion, not avail much; for it is not easy to conceive how the
motion of a subtile fluid, or the vibration of a nerve, can cause
sensation.
Nor are the internal senses, as they are generally called, namely,
memory, and imagination, any better explained on this supposition;
for we cannot conceive how this nervous fluid is stored up and
propelled by the will.
After all, I think we must confess, that this subject is still
enveloped in obscurity. One observation is worth making, namely,
that our sensations have not the smallest resemblance to the
substance or impression, which causes them; thus the sensation
occasioned by the smell of camphor, possesses not the smallest
resemblance to small particles of camphor floating in the
atmosphere; a sensation of pain has no similitude whatever to the
point of a sword which occasions it; nor has the sensation of
sound any resemblance to waves or tremors in the air. In our
present state of knowledge, therefore, all that we can say, is,
that nature has so formed us, that when an impression is made on
any of the organs of sense, it causes a sensation, which forces us
to believe in the existence of an external object, though we
cannot see any connexion between the sensation and the object
which produces it.
But though philosophers were certainly blameable for assuming an
unknown cause, to account for various phenomena, yet later
experiments would seem to prove that even the conjectures of
Newton were not founded on slight grounds. His idea that the
diamond was inflammable, has been confirmed by various
experiments: and that there exists in nature a subtile fluid,
capable of pervading with ease the densest bodies, the phenomena
of electricity would seem to show, and some late experiments
render it by no means improbable, that this fluid or influence,
acts a conspicuous part in the nervous system. That it exists in
great quantity in animal bodies, is evident on gently rubbing the
back of a cat in the dark; and it would seem that, in some
instances, it may be given out or secreted by the nerves. We shall
afterwards see that the seat of vision is a delicate expansion of
a large nerve which comes from the brain, and is spread out on the
bottom of the eye; and flashes of light, or a kind of sparkling,
is often seen to dart from the eyes of persons in high health, and
possessed of much nervous energy. These luminous flashes are very
apparent in the dark in some animals; such as the lion, the lynx,
and the cat; and it is difficult to account for this appearance
unless we suppose it electrical.
The experiments of Galvani and others, have however proved beyond
all doubt, that this fluid, when applied to the nerves and
muscles, is capable of exciting various sensations and motions. To
produce this fluid by the application of two metals, it is
necessary that one of them should be in such a situation, as to be
easily oxidable, while the other is prevented from oxidation. If a
piece of zinc be put into water, no change will take place; but if
a piece of silver be put along with it, the zinc will immediately
oxidate, by decomposing the water, and a current of electricity
will pass through the silver. If the upper and under surfaces of
the tongue be coated with two different metals, one of which is
easily oxidable, and these be brought into contact, a sensation is
produced resembling taste, which takes place suddenly, like a
slight electrical shock. This taste may likewise be produced by
applying one part of the metals to the tongue and the other to any
part of the body deprived of the cuticle, and bringing them in
contact.
The sensation of light may be produced in various ways; such as by
applying one metal between the gum and the upper lip, and the
other under the tongue; or by putting a silver probe up one of the
nostrils, and a piece of zinc upon the tongue; a sensation
resembling a very strong flash of light is perceived in the
corresponding eye, at the instant of contact.
But the experiments which tend most strongly to prove what I have
hinted, are made in the following manner. Lay bare a portion of a
great nerve leading to any muscle or limb of an animal, for
instance, the leg of a frog separated from the body. Touch the
bared nerve with a piece of zinc, and the muscle with a piece of
silver, and strong contractions take place the instant these
metals are brought into contact. The same effect may be produced
by placing a piece of silver on a larger piece of zinc, and
putting a worm or a leech on the silver; in moving about, the
instant it touches the zinc it is thrown into strong convulsions.
These phenomena have been clearly proved to be electrical; for by
a number of pieces of these metals, properly disposed, strong
shocks can be given, the electrometer can be affected, a Leyden
vial charged, the electric spark seen, and combustible bodies
inflamed.
Some animals likewise possess the power of accumulating this
influence in a great degree; for instanc the torpedo, and
electrical eel, which will both give strong shocks; and if the
circuit have a small interruption a spark may be seen, as was
shown by Mr. Walsh. On dissecting these fish, Mr. Hunter found an
organ very similar to the pile of Volta; it consists of numerous
membranaceous columns, filled with plates or pellicles, in the
form of thin disks, separated from each other by small intervals,
which intervals contain a fluid substance; this organ, like the
pile of Volta, is capable of giving repeated shocks, even when
surrounded by water.
It is not absolutely necessary to use two metals to produce the
galvanic phenomena; for if one side of a metal be made to oxidate,
while the other is prevented from oxidation, these appearances
will still be produced. It is not indeed necessary to use any
metal; for a piece of charcoal, oxidated in the same way, produces
galvanism; so does fresh muscular fibre, and perhaps any substance
capable of oxidation. The most striking circumstance in galvanism,
is, that it accompanies oxidation, and is perhaps never produced
without it. But oxidation is always going on in the body by means
of respiration and the circulation of the blood. We shall
afterwards find reason to believe, that the oxygen, received from
the atmosphere by the lungs, is the cause of animal heat, and
probably of animal irritability; and it is perhaps not
unreasonable to suppose, that the nervous influence or electricity
may be separated by the brain, and sent along the nerves, which
seem the most powerful conductors of it, to stimulate the muscles
to action.
What the nature of the electric fluid is, we are ignorant; some
galvanic experiments have led me to suppose that it may be
hydrogen, which when combined with caloric appears in the form of
gas, but when pure, or perhaps in a different state, may be
capable of passing through solid bodies in the form of
electricity.
Having given this short view of the human body, considered as a
machine composed of bones, muscles, and nerves, I shall proceed to
state the different subjects which I shall consider in this
course. It is extremely difficult to begin a course like this; for
we must either enter abruptly into the middle, or the outset must
be in some measure tedious and dry. I have chosen however rather
to hazard the latter appellation, with respect to this lecture,
than to enter more abruptly into the subject, in order to make it
more entertaining. As we proceed, I trust you will feel an
increasing interest in the subject; and, I think I may venture to
promise, that this will be found the least entertaining lecture in
the course. The subjects will be illustrated by experiments, in
order to render the deductions more striking.
I shall next proceed to consider the phenomena of respiration, and
animal heat; after which I shall explain the circulation of the
blood; and the phenomena of digestion and nutrition. I shall then
examine, more minutely than has been done in this lecture, the
connexion of man with the external world, which will lead to a
discussion of the different senses; vision, hearing, smelling,
tasting, and feeling.
When these subjects have been discussed as fully as our time will
allow, I shall examine, at considerable length, the manner in
which the powers that support life, which have been improperly
called by physiologists, the nonnaturals, act upon the body. This
will naturally lead to a fuller explanation of the system which I
have attempted to defend, in my lecture on health. And, after I
have fully explained the laws by which the irritable principle is
regulated, I shall proceed to show, how those variations from the
healthy state, called diseases, are produced; I shall point out
the difference that exists between the debility which is brought
on by the diminished action of the powers which support life, and
that which results from their too powerful action; I shall then
inquire into the nature of diseases of increased excitement; and
after having shown how the undue action of the powers which
support life, operates in producing disease, I shall endeavour to
lay down such rules for the preservation of health, as are the
result of reasoning on these subjects, and are also confirmed by
experience.
[1] [FIGURE] Suppose AC to be a lever, held in equilibrio by the
force B and weight W, then the whole momentum exerted at B must be
equal to that at W, but the forces will be different. For B x AC = W
x AB, and if AC = 10AB, then a force equal to ten times the weight to
be raised must be exerted by the muscle. Hence we see, that in the
actions of muscles there is a loss of power, from their insertions
being nearer the fulcrum than the weight. For example, suppose the
deltoid muscle to act and raise a weight of 55 lb.: the weight of the
arm is 5 lb., and the distance of its insertion is only 1/3 of the
arms length, hence the force exerted must be (55 + 5) x 3 = 180 lb.
[FIGURE] But by this contrivance we gain a greater extent of motion,
and also a greater velocity, and both with less contraction. Let A be
the centre of motion, or articulation; B the insertion of a muscle,
and AC the length of the lever or bone; then, by a contraction only
equal to B_b_, C is carried through C_c_, which is to B_b_ as AC to AB.
It is obvious also, that the velocity is greater, since C moves to _c_
in the same time as B to _b_.
A loss of power is likewise occasioned by the obliquity of the
muscular action, and the oblique direction of the fibres.
For, in this case, there is a compound of two forces, and a
consequent loss of power: for the forces are proportioned to the two
sides of a parallelogram, but the effects produced are proportioned
only to the diagonal.
LECTURE II.
RESPIRATION.
In the last lecture I took a short view of the human body, as a
moving machine, regulated by the will. We shall now proceed to
examine some of its functions more particularly.
I need not tell any of my audience, how necessary air is to the
living body; for every person knows that we cannot live when excluded
from this fluid; but, before we can understand the manner in which it
acts on the body, we must become acquainted with some of its
properties.
That the air is a fluid, consisting of such particles as have little
or no cohesion, and which slide easily among each other, and yield to
the slightest force, is evident from the ease with which animals
breathe it, and move through it. Indeed from its being transparent,
and therefore invisible, as well as from its extreme tenuity, and the
ease with which bodies move through it, people will scarcely believe
that they are living at the bottom of an aerial ocean, like fishes at
the bottom of the sea. We become, however, very sensible of it, when
it flows rapidly in streams or currents, so as to form what is called
a wind, which will sometimes act so violently as to tear up the
strongest trees by the roots, and blow down to the ground the best
and firmest buildings.
Some may still be inclined to ask, what is this air in which we are
said to live? We see nothing; we feel nothing; we find ourselves at
liberty to move about in any direction, without any hindrance. Whence
then comes the assertion, that we are surrounded by a fluid, called
air? When we pour water out of a vessel, it appears to be empty; for
our senses do not inform us that any thing occupies the place of the
water, for instance, when we pour water out of a vial. But this
operation is exactly similar to pouring out mercury from a vial in a
jar of water, the water gets in and supplies the place of the
mercury; so does the air which supplies the place of the water; and
this air will prevent water from rising, or filling a vessel which
contains it.
Hence we see that air possesses similar appearances of
impenetrability with other matter: for it excludes bodies from the
space which itself occupies.
Air being therefore material must have weight; and we shall
accordingly find, that a quart of it weighs about fifteen grains. But
a quart of water weighs about two pounds; this fluid therefore is
nearly a thousand times heavier than air.