Chapter VII: Sixth Day (3)

The next part of the blood, or coagulating lymph, is of the greatest importance. This constituent part becomes apparent, when blood is drawn from a vein into a cup, from its power of spontaneously coagulating into a solid mass, which appears red from a mixture of red particles: the color of the lymph however is transparent. This coagulation of the blood differs very essentially from the coagulation of inanimate substances, and is considered by many physiologists to be the last exertion of a living principle, which the blood is supposed to possess. This opinion, although not capable of absolute proof, is rendered extremely probable from a variety of facts, and by none more than the analogy between the coagulation of the blood, and the contraction of the muscles at death. These two actions appear to be influenced in some degree by the same causes. Thus, sudden death from lightning, or a blow upon the stomach, prevents the muscles from becoming rigid after death, and prevents also the coagulation of the blood. Under these circumstances it remains fluid. Besides violent death, several circumstances influence its tendency to coagulate and become solid, such as a great loss of blood--inflammation--pregnancy in females, and other causes.[191]

The third important part of the blood is the serum. This is limpid like water, and remains permanently fluid, unless certain substances are employed to coagulate it, such as alcohol, alum, or a certain degree of heat. It dilutes the other parts of the blood, so as to reduce the whole to a proper state of fluidity. It is secreted, or naturally separated from the blood, and poured out by exhalent vessels in various cavities and parts of the body, as the chest, abdomen, cellular membrane, &c. It facilitates the easy motion of the various organs upon each other, and, when accumulated in large quantities, forms the fluid of dropsies.

Besides these constituent parts, a quantity of water always circulates with the blood, varying according to the quantity of fluids taken in, and regulated in its proportion by the kidneys. Thus if a large quantity of water is taken into the stomach, particularly if it contain a little spirit in the form of punch, the kidneys are stimulated to an increased action, so as to separate from the blood the redundant quantity. A variety of other substances also are occasionally introduced into the blood, along with the aliment, alkaline substances producing their effect upon the nature of the urine, rhubarb on bile giving it a yellow color, and turpentine or asparagus altering its odor; all these substances, before passing off by urine, must have been mixed with the blood, from whence the urine is formed, being in fact its excrementitious part.[192]

It is necessary for the blood thus formed, to pass to every part of the body, that it may be converted into the nature of these parts, and thus become subservient to their growth; that fluids, serving important purposes in animal bodies, may be separated or secreted from it; and that the temperature of the body may be equably maintained. The blood, however, has no power of motion in itself; if it be not propelled by certain parts of the body, it remains quiescent like any extraneous fluid.

In two very numerous classes of animals, insects and zoophites, the motion of the blood is very simple; they are nourished like vegetables, by the absorption of the fluid, which is prepared in their alimentary canal, and have no circulation properly so called.

But in man, and the higher orders of animals, a complex apparatus for the motion of the blood becomes necessary, consisting of an heart, arteries, and veins. The _heart_ may be considered as the chief agent in circulation, the general reservoir, and source from whence the blood flows. It is composed of two principles, one a principle of reception, the other a principle of propulsion. That cavity of the heart, which is called its auricle, receives the blood from the veins; the cavity called its ventricle, propels it through the arteries.

Although the heart in all animals is formed on the same general principle, and for the same purpose, yet the economy of some animals admits of a greater simplicity in the conformation of this organ, than others. The most simple kind of heart is composed of one cavity, with a tube entering into it, by which it receives the blood, and another passing out of it, by which the blood is conveyed over the body. The next simple heart is composed of two cavities, an auricle, which receives the blood, and propels it into a ventricle, which diffuses it over the body. Another kind of heart is composed of three cavities; two auricles, and one ventricle; one auricle receiving the blood from the lungs, the other from the body generally; the blood from these two sources is mixed together in a single ventricle. This structure we find in some amphibious animals, in which it is not necessary that the blood should circulate with so much influence from the oxygenous part of the atmosphere, as in other animals. Accordingly we find the heart adapted to transmit only one half of the blood through the lungs at each circulation, whilst in more perfect animals the whole mass passes by this route. The last kind of heart is formed of four cavities, two auricles and two ventricles, and is the most perfect apparatus as it is found in man, and quadrupeds generally.--It must, however, be considered as composed of two distinct parts, or two simple hearts adhering together, and performing distinct parts of the circulation; and one part intended to receive the blood from the body, and circulate it through the lungs; the other part to receive the blood from the lungs, and propel it over the rest of the body. It is better suited to the economy of some animals, as the cuttle fish, that these parts should be separated to a considerable distance from each other. The reason why the heart is formed of two parts in most animals is, that it is necessary that the blood should receive the impulse of the heart twice, first to propel it through the lungs, next to propel it over the rest of the body.

The blood is conveyed from the heart to every part of the body, by means of elastic tubes, called _arteries_. These arise from the ventricles of the heart by two large trunks, which branch out in every part of the body, into arteries of great minuteness, conveying the blood from the heart to its most distant parts, so that it is impossible to wound any part of the body with the finest point, without opening one of these vessels. This gives a good idea of their minuteness.

From the minute termination of the arteries, begins a second set of vessels, the _veins_, which, having a contrary course, return the blood from every part of the body into the auricles of the heart.

The larger arteries and veins, near the heart, differ very much from each other in their structure and action. This difference, however, does not descend to their minute ramifications, which must be considered as having the same structure, and performing the same office, the one passing into the other by such imperceptible degrees, that we cannot mark where the one terminates or the other begins.

If we consider these tubes as subservient to the circulation of the blood, we shall see the necessity of certain principles entering into their structure. As the blood is forcibly thrown from the heart, these vessels must be distended; one of their properties therefore, must be a capability of being distended, which is given to them by elastic matter entering into their composition. As the vessels, however, are not to remain in a distended state, a power of reaction is added, which arises also out of their elasticity, and assists in propelling the blood forwards.

Thus the elastic matter allows the vessels to be distended to a certain degree, and also reduces them to a smaller size. But it is necessary that the heart shall be assisted considerably, in the circulation of the blood, by a contractile power of the vessels themselves; and the same quantity of blood is not to circulate in the same body at all times, for animals are liable to frequent injuries, by which the quantities of blood in their bodies may be very suddenly reduced. Hence the vessels have given to them a further power of contraction to assist the heart, and accommodate themselves, under certain circumstances, to a smaller quantity of blood. For this purpose, a muscular structure is added to them, which is present in largest proportion in the smaller arteries; by this means, they are enabled so far to withstand the power of the heart, as to shut their cavities, and prevent the escape of blood when divided, forming one of the means by which the effusion of blood is spontaneously checked in living animals. And it may be remarked, that this power, for the purpose of self-preservation, is extended to larger arteries in the brute creation; for Mr. Hunter found, that the flow of blood from the large artery in the neck of an ass was checked by an exertion of this power, whilst every one knows that its division in man is fatal.

Besides these parts, arteries have an internal lining, which is perfectly smooth, and of considerable density, that the blood may circulate with as little resistance, and be contained as completely as possible within its proper channels.

The same observations will apply to the veins, though some of their properties are less strongly marked. They possess an elastic power capable of distension and reaction, a muscular structure endowed with contractility, and an internal lining over which the blood circulates with as little resistance as possible. By these powers the blood is circulated through every part of the body with great velocity. According to the best calculations, the heart alone exerts a power equal to the pressure of 51½ pounds, which propels the blood through the arteries at the velocity of 149 feet in a minute; in which time it expels from its cavities about 160 ounces.

Thus all animals are provided with an organ for propelling the blood, by certain channels, to the different parts of the body; but, as the functions of these parts are various, they require to be visited by very different proportions of blood, according to their activity or powers of life. Some parts of the body may be said to be inert, and merely possessed of a principle of life, to connect them with the other organs of the body, as parts of a living system, and to enable them to go through certain processes in their healthy and diseased states. Other parts are formed for active functions, and possess great sensibility. It is accordingly observed, that a smaller quantity of blood is distributed to bones, tendons, and similar inert parts, than to muscles and glands, whose exertions are more considerable.

This then is the general apparatus in perfect animals, by which the blood performs its circulation through the various parts of the body, but during its course it is subject to constant exhaustion from various sources. It is converted in its passage into the nature of all the component parts of the body, and has the different secreted fluids derived from it, and these processes go on with more activity in a young, than an adult person: hence we see the necessity of a constant supply of materials to the blood, and this in the greatest proportion at an early period of life.

Animals are furnished with the means of this supply, by their power of converting animal and vegetable substances into the nature of blood, by a process called _digestion_. Some animals are led by their nature to live on vegetable food, others on animal food only, whilst others can subsist on either, or any mixture of both.[193] The digestive powers of man fit him for any proportion of animal or vegetable foods, and are the most perfect of all animals. Other creatures may be said to be confined to a certain district, but the curiosity of man is to lead him over the whole world, and frequently place him in situations where only one kind of food is attainable.

The first change which takes place in the food, in order that it shall be converted into the nature of the blood, is its division into smaller parts, by the teeth or gizzards of animals. It is then passed into the stomach, where it remains for some time exposed to the action of a fluid, formed in the stomach, which is called gastric juice. This possesses a very strong power of coagulating and dissolving various animal and vegetable substances. As far as we know, it acts on the principle of any other solvent, for it produces the same change in substances out of the body, or even within the body after death. It frequently happens, for instance, when a person has been killed, by accident, in full health, that, on inspection, the stomach is found dissolved, and reduced to a gelatinous mass in several parts, arising from the action of the gastric juice, which had been formed in it before death. The gastric juice, however, cannot act upon living substances: hence the stomach resists its action, and worms sometimes reside and are even generated in the stomach. Every substance capable of being acted upon in the stomach, is reduced, by the solvent power of the gastric juice, into a pulpy mass, which has been called chyme, the exact chemical properties of which have not been ascertained; in this state it is by degrees transferred into the beginning of the small intestines, where it is mixed with the bile and pancreatic fluid, and undergoes a change into a milky fluid, which is called chyle. It is then diffused by an undulating motion of the intestines over their inner surface, that it may be absorbed, and carried into the general mass of blood.

As far as has yet been ascertained by experiment, the chyle of animals, most opposite to each other in their food, structure, and habits of life, is so much alike as to have no distinguishable difference. The chyle of a Dog, or Wolf, differs in nothing from that of a Sheep or an Ox. This would appear surprising, were it not ascertained that almost every alimentary matter undergoes a chemical change before it is converted into chyle, and that the ultimate analysis of either animal or vegetable matter presents us with the same elements as those of the blood, which, though only three or four in number, are capable of forming the various substances of which the body is composed, by combining with each other, and in different proportions. There is, however, this difference observable in the chyle, that in reptiles and insects it is transparent like lymph.

The lacteals are the vessels by which the chyle is absorbed from the intestines: they form small processes on the internal surface of the intestines like the pile of velvet, which are hence called villi. A small portion of chyle being received into their open mouths, is propelled by successive contractions of these vessels into their large trunk, the _thoracic duct_, from whence it is poured into a great vein near the heart, and, by circulating through the lungs, probably receives its final change into blood; and this change would seem to be easily effected, as the chyle already possesses the principal properties of blood, being formed of particles swimming in a thinner fluid, and having a power of coagulating spontaneously.[194]

This is the apparatus by which the food is digested in man so as to replenish the blood; but the digestive organs of different animals exhibit considerable varieties, some being more simple, others more complex in their structure, adapted to the kind of food with which the animal is nourished. Ruminating animals, or animals which chew the cud, such as the Cow, have several stomachs, and the food undergoes mastication several times, at each time being passed into a different stomach, before being finally acted upon by the gastric juice, after which it is transmitted through a long tract of intestines. This is an example of the most complex digestive organ fitted to act upon hard and fibrous food, which must be subjected to the action of several menstrua preparatory to its being acted upon by the gastric juice.

In birds who live on grain as has been noticed, we meet with a different apparatus to prepare it to be acted upon by the gastric juice. The food first passes into the crop, which forms a kind of reservoir from whence it may pass by degrees into the gizzard, by which the grain is ground into small particles, before it is transmitted into the stomach: and it is surprising with how great power the gizzard acts for this purpose. The Abbé Spallanzani introduced a garnet, which is a very hard and angular stone, into the gizzard of a Wood-Pigeon, and, in the course of a day, it was ground perfectly smooth, by the action of the gizzard. He also introduced a leaden ball stuck full of tin points, and another with fine lancets, into the gizzard of a Turkey, and in about 18 hours, the whole of the points were rubbed down. The gizzard also possesses an amazing power of compression. Raumeur introduced into the gizzard of a Turkey tubes of tinned iron, seven lines in length, and two in diameter, closed with solder at each end; some were indented by the action of the gizzard, and others crushed flat. Similar tubes, introduced into the teeth of a vice, required the weight of about 440 lb. to produce the same effect. The gizzard thus reduces into small particles whatever food the animal selects, that it may be more readily acted upon by the gastric juice in the stomach; for the gastric juice acts like any other solvent, and therefore acts most advantageously when the food is reduced into small parts.--The digestive organs of some of the lower orders of animals form a striking contrast to these. In the most simple apparatus with which we are acquainted, the stomach and the intestines are composed of a simple bag which has but one opening, which serves both to receive the food, and discharge the excrement. It composes in fact the whole bulk of a fresh-water Polypus. In these animals the chyle is absorbed by small vessels in the sides of the bag, and is conveyed to every part of the body.

Thus we find that the supply of materials to the blood is commensurate to its exhaustion, that in young animals where a more active process of formation is going on, a larger proportion of food is requisite, and more chyle formed; this, however, is not all that is necessary to prepare the blood for its important purposes within the body. The blood, by passing through the various parts of the body, is so changed by the abstraction of certain properties, as to render it unfit for circulation, which implies the necessity of an organ, which may restore to the blood its requisite qualities. This office is performed by _respiration_, that function in animals by which the blood receives the influence of atmospherical air.

There is a great variety in the structure of the organ for exposing the blood to the air, suited to the mode of life in different animals. In man and quadrupeds generally the lungs serve this purpose; they are composed of a number of blood vessels spread out upon minute air cells, which communicate with and receive the air by means of the trachea or windpipe, in consequence of the expansion of the chest by certain muscular powers. These vessels and cells are connected together by cellular membrane, so as to form a spongy mass called lungs, which are commonly placed in the chests of animals.--But besides this kind of organ, which in birds is very large, they have air bags, or appendages to the lungs, diffused through various parts of the body; even some of their long bones contain nothing but air, and communicate with the lungs. It was from a knowledge of this fact that Mr. J. Hunter made a Turkey breathe by its wings, by making an opening into their large bones, and closing the animal’s mouth.

In Fish, the gills serve the purpose of lungs. They are composed of a number of processes arising from cartilages, having distributed upon them minute blood-vessels, which receive the influence of air contained in water: and hence distilled water, which contains little air, destroys fish, in the same manner as the exhausted receiver of an air pump does a breathing animal.

There is another mode of conveying air for the use of the blood in many insects, by means of a number of tubes or spiracula: these receive the external air, and, by ramifying in the body of the animal, convey its influence to the blood. Thus these animals may be said to respire like vegetables, throughout the whole of their surface, by vessels which introduce the air at different points into their bodies. In some insects the rectum forms the principal organ of respiration, and, in the class of animals called Zoophites, there are no visible organs of respiration.

These different modifications, in the respiratory organs of the higher and lower orders of animals, are all formed with the same intention, viz. that the blood may be exposed more or less to atmospherical air. In consequence of this the blood undergoes a process similar to combustion, which extracts from it a part of its carbon, in the form of carbonic acid, and by this means increases the relative proportion of its remaining elements. The inspired air at the same time is deprived of a part of its oxygen, which is the elastic fluid which commonly supports respiration. All the corresponding effects produced upon the blood are not yet fully explained. But by this means the color of the blood is changed from a dark to a florid red, it acquires the power of exciting the action of the heart, and is fitted for its various purposes within the body.[195] By these organs, respiration is performed more or less extensively in the different orders of animals, corresponding in a great degree, to their activity, digestive powers, and the heat maintained in their bodies. Birds, whose extensive respiratory organs consume a larger quantity of air, are capable of greater exertion; make more frequent meals than quadrupeds, and maintain a superior temperature. Quadrupeds hold a middle place between birds and reptiles. Respiration appears in the class of reptiles, as Frogs and Toads, to be a subordinate function only; they can exist without it nearly as long as they please; at the same time they make very long fasts, and the heat of their bodies is more variable and lower than quadrupeds; hence they are called cold blooded animals. Their other habits accord well with their organs of respiration. They generally live in impure air, their motions are languid, and they pass a great part of their existence in a state of torpidity.

A subordinate use of respiration in most animals, is the formation of the voice: for this purpose there are membranes stretched across the narrow part of the windpipe, which are thrown into a state of vibration by the current of air: the vibrations thus produced, being modified by other accessory parts, produce the voice. In many animals, however, it is produced by a very different mechanism. Some animals employ the friction of certain elastic parts of the body, as Grasshoppers and Crickets; others employ the vibration of certain parts in the air, whilst others impress a rapid motion on portions of air inclosed in certain parts of their bodies.

There is a particular part of the heart in man, intended merely to propel the blood, which passes through the lungs to receive the influence of the air; this is the right ventricle; from whence the blood passes, by the pulmonary artery, through the minute vessels expanded on the air cells, and is changed from a dark to a florid color: it is then returned back to the left ventricle, by the pulmonary veins, and is propelled over the rest of the body, where it is again changed (by the abstraction of certain properties) to the dark color peculiar to venous blood: the blood is lastly conveyed by the veins to the right side of the heart from whence it set out, having passed through two circles.

The blood thus prepared by the lungs for circulation, passes in different quantities to different parts of the body, according to their activity, and has various fluids formed from it, which are called secreted fluids, as gastric juice, milk, bile, &c. The parts of the body forming many of these fluids, are very peculiar in their structure, and are called _glands_. They consist in an arrangement of vessels, endowed with a mode of action, with which we are unacquainted, by which the component parts of the blood are disposed to enter into new combinations, and to form compounds differing from the blood itself. Thus the vessels are arranged on the inside of the stomach, in such a way, as by their action to form gastric juice from the blood; on the same principle, milk is produced from the blood which circulates in the breast, or bile in the liver. As gastric juice, milk, and bile, differ very much from each other in their properties, we must infer, that there is a considerable variety in the action, by which these vessels form these fluids from the blood; and this is necessarily connected with a variety in arrangement, which is the case in all the glands of the body. In one gland, for example, the blood-vessels form a minute net-work; in another, are convoluted at their extremities; in a third, a large branch suddenly divides into a number of small branches, like the hairs of a painter’s brush; in a fourth, they are disposed in an arborescent form, each gland differing from every other in the mode of distribution of its blood-vessels, and forming different products from the blood.

The substances formed by many of the glands of the body, are applied to useful purposes, within or without the body. An instance of the former we have in the bile formed by the liver, or the gastric juice formed by the stomach; and of the latter, in the milk.--Other secreted fluids are rejected as excrementitious: the best example of this is the urine formed by the kidneys. This gland separates from the blood a great variety of substances, which might otherwise prove noxious by circulating along with it; many of these have occasionally very curious chemical properties, and under a certain state of the body, the altered secretion of this organ is very remarkable, in as far as it produces a large quantity of a familiar substance, which in this instance is composed within the body. In the disease called diabetes, for example, a patient sometimes makes four or five gallons of urine in the 24 hours, in which is dissolved a considerable quantity of matter, like common sugar or treacle, probably to the amount of two or three pounds.

Besides these fluids formed from the blood, each by an appropriate glandular apparatus, there are watery fluids constantly secreted in various parts of the body; and, that these may not accumulate, or remain after they have performed their office, it is necessary for the body to be furnished with vessels, whose powers of removal may keep pace with the deposition of these fluids. This introduces the system of vessels called _absorbents_, which are distinct in their office and nature from the blood-vessels, and are widely diffused over the whole body. In every part of the body a limpid fluid is thrown out for the purpose of easy motion, moistening the cellular membrane, which connects the various parts of the body to each other, and lubricating the contents of all the cavities of the body; this fluid is thrown out in the form of vapor by the exhalents, which belong to the arterial system, whilst the lymphatic absorbent vessels, by their action, remove what is not convenient for the function of the part; and these two actions, of deposition, by the exhalents, and absorption, by the lymphatics, go on during health, so nicely balanced, that when we open into any of the great cavities of the body, as the belly or chest, the quantity of fluid we find is extremely small. When, however, the balance between these two orders of vessels is destroyed, when the exhalents throw out more fluid than usual, and the lymphatics only absorb their natural quantity; or the exhalents deposit their natural quantity, whilst the lymphatics absorb less than natural, accumulation of water in the cellular membrane, or great cavities of the body, takes place, and produces dropsies.

There is another set of vessels, which have been already mentioned, a part of the same system of absorbents, which from their office of absorbing a white fluid, the chyle, have been denominated lacteals; these arise from the inner surface of the intestines, in great numbers, and convey the chyle into the general mass of blood.--Whilst the minute beginnings of the lacteal vessels, from the internal surface of the intestines, is a matter of ocular demonstration, we have only presumptive proof of the origin of the lymphatics, which make the greatest part of the absorbent system. We have, however, good grounds for concluding, that they arise from every external and internal surface of the body. We find, for example, that certain remedies, as mercurial ointment, or turpentine, rubbed on the skin of any part of the body, produce effects on distant parts; the mercury by removing affections of various parts of the body, the turpentine increasing the flow of urine, and giving it a peculiar odor: these effects are explained by presuming the absorption of these substances, by the lymphatics, arising from the surface of the skin. We have further proof of this from the occasional absorption of watery fluids, under peculiar circumstances. Sailors at sea, in want of fresh water, have quenched their thirst by dipping their clothes in salt water, and applying them to the surface of the body, from which only the elementary part was absorbed by these vessels. A jockey, after reducing himself to a great degree has become in a short time too heavy to ride his match, merely by drinking a glass of wine, which had stimulated the absorbents of the skin to take up a large quantity of aqueous matter from the air. Or a person gibbeted alive, has been observed to make a considerable quantity of urine as long as he lived, without any liquid being taken by the mouth. These are all considered as evidences that the lymphatic absorbent vessels arise from every external surface of the skin, and are capable of taking up substances applied to them.

We find next that water accumulated in the large cavities of the chest or abdomen, or underneath the skin in the cellular membrane, of every part of the body, is occasionally removed from these situations, by remedies which have the power of increasing the action of the absorbent vessels. We hence conclude, that these vessels arise from every internal part, and are, in short, widely diffused over the whole body, though their beginnings are too minute to be detected by any mode of examination with which we are acquainted.

The absorbent vessels, from whatever part they arise, terminate in the blood-vessels, principally by one vessel or trunk, which is called the _thoracic duct_. This commences in the cavity of the abdomen, passes through the chest on the right side of the spine, and, at length, enters a large vein situated on the left side of the neck. Through this vessel, besides the fluids taken up in various parts of the body, the whole of the nourishment from digested aliment passes into the blood; it may therefore be said to be the most important vessel in the body,[196] and it is situated in one of the safest positions in the body, so that an injury done to it is a very rare occurrence.

Thus the absorbent system is formed of two sets of vessels, having the same structure, the same absorbing office, and the same termination, but differing in the fluids they convey, and the parts of the body they occupy. The one widely diffused over the whole body, and from their office of usually absorbing limpid fluids, called lymphatics; the other arising only from the intestines, and denominated lacteals, from the milky whiteness of the chyle they absorb.

Thus far the absorbent vessels have been described, as employed in taking up fluids only. The action of the absorbent system, however, is not considered as confined to the fluid parts of the body; there are a variety of instances, in which the most solid parts appear to be removed by the absorbents. Thus when a tooth is extracted, or drops out in old age, its bony socket is removed by the action of the absorbents. The pressure of a pulsating tumor, called aneurism, against the ribs, or thigh bone, has produced their removal in the same way. These are considered as instances of solid matter being removed by the absorbent vessels, from internal parts of the body, without any external opening. It is, however, a matter of doubt, which we cannot at present discuss, whether a bone is broken down by the absorbents themselves, so as to be removed in small particles; or whether, as is more probable, its presence or irritation (as an extraneous body) produces the secretion of a fluid, similar in its properties to the gastric juice, by which it is first reduced into minute particles, or entirely dissolved, so as thus to enter the absorbent vessels.

Another important part of the office of these vessels, is to model the shape of the body, and to concur with the action of the blood-vessels in regulating its growth. For the human body does not, like a marble statue, constantly contain the same identical particles in its composition. As the stream of a river is formed of a constant succession of aqueous particles, sometimes increasing, sometimes diminishing its natural bulk; so the human body is constantly undergoing an imperceptible change of parts. The absorbents, by their action, remove exhausted particles, whilst the arteries form from the blood an adequate supply of new parts. When these two powers are equal, the body continues of the same bulk; when from disease or contingent circumstances, the one or the other predominates, the body increases in growth, becomes corpulent, or emaciated.

Thus we have seen a variety of organs necessary to carry on the functions of perfect animals: these, however, are inert, and incapable of motion in themselves. Hence a _nervous system_ becomes requisite, which may excite and influence the whole. We find in man, and quadrupeds generally, the nervous system placed principally in the brain and spinal marrow; from these sources, the nerves are distributed like white cords, and pass in various proportions to the different parts of the body, conveying the excitements of the brain.

One of the most important excitements conveyed from the brain, through the medium of the nerves, is volition; by this means the muscles become obedient to the will, and perform the voluntary actions of animals. If, for instance, I wish to take up a pen, I exert my volition towards the action, and the consequence of this is, that the muscles employed in the action, are stimulated to contract, from a peculiar excitement being conveyed to them from the brain, through the medium of the nerves. We are totally ignorant, however, of the state of the brain, whilst giving out the excitement, or the change which takes place in the nerves whilst conveying it. We know, however, that the brain may be rendered incapable of giving rise to the excitement, and it may be arrested in its progress down the nerves by artificial means. If a ligature be applied upon a nerve by tying a piece of thread round it, the nerve is rendered incapable of transmitting the excitement, so as to produce motion in muscles. The same state is frequently produced in the brain and nerves, by the disease called palsy, or by fractures of the skull. There are also various excitements passing from the brain to the vital organs of the body, whose actions are not regulated by the will, and are therefore called involuntary, or automatic actions, as circulation, parturition, &c. Thus if a person have ever so strong a desire, he cannot make his heart beat more frequently; nor can he prevent it from beating more frequently, if any one should put him in bodily fear; although the heart is formed of muscular flesh, similar to the muscles, which he can command in his arm. The reason of this is, that the nerves of the heart cannot convey the influence of volition; for the wisest reasons the heart acts without it.

It is also necessary for various influences to be communicated from external objects to the brain, to keep up a correspondence between animals, and the material world around them, and to communicate those impressions from which the brain is afterwards to carry on its functions. As the parts formed for this purpose differ from ordinary parts of the body, in having a larger share of nervous influence given to them, they have been called the _organs of sense_, which in an anatomical point of view, may be said to be five in number, the eye, the ear, the tongue, the nose, and the skin.

In the _eye_, we discover a most accurate optical instrument, adapted to converge the rays of light at its posterior part. It is composed of a spherical box, containing transparent media of different densities, by which the rays of light are conveyed to a point, so as to impress a minute image of the visible appearance of external objects upon the retina or expansion of the optic nerve, by which the impression is conveyed to the brain, so as to bring us acquainted with external objects.

The _ear_ is formed to receive impressions from bodies in a state of vibration, which are conveyed to the brain by an apparatus composed of various substances, and eminently calculated to transmit the slightest tremors. The vibrations of the air, for instance, first strike the drum of the ear; are thence communicated to a delicate chain composed of four minute bones. By these the vibration is increased, and transmitted to a fluid, contained in several small winding canals, in which the delicate filaments of the nerves of hearing are arranged, so as to transmit the impressions they receive from the surrounding fluids, and produce in the brain the perception of sound; these two senses, by the infinitely varied modification of their impressions, convey a prodigious supply of materials for the action of the mind.

The organ of _touch_ is next in point of importance; it has its seat in the extremities of the nerves distributed over the skin, and is the only sense which belongs to every class of animals. This organ gives rise to sensations, which have no natural alliance with each other. By this sense we compare different degrees of temperature with each other; from this we derive our idea of distance between bodies; of their tangible figure, of their roughness, smoothness, hardness, and other qualities, from the relative position with respect to ourselves, or the degree or kind of resistance they offer. And, when man has been deprived of his communication with many external objects, by the loss of vision, we find the organ of touch gradually encroaching upon the function of the eye, and from attention to its finer impressions, becoming, through the education of necessity, a much more extensive source of information. As an instance of this, I may adduce Mr. Gough, who can accurately distinguish the color and character of flowers, by the nice sense of touch possessed by the tip of the tongue.

The other senses may be said to be of less importance. The _nose_ affords a passage for the air to the lungs, and is impressed by the odorous particles of bodies diffused through it, and, whilst it thus occasionally administers to our gratification, it gives us notice of the presence of those aeriform fluids which are noxious to respiration. Like the organ of _taste_, which is impressed by sapid bodies, it has a peculiar sympathy with the stomach; thus the taste, or smell, of any disagreeable substance, very commonly excites sickness and vomiting.

Thus each of the organs of sense are formed in a peculiar manner, and are supplied with nerves of a peculiar structure, which are capable of being excited by certain impressions only, so as to give rise to sensation. The odorous particles of bodies, for instance, if applied to the nerves of the nose, excite an impression, which, when conveyed to the brain, gives rise to the perception of smell; but, every one knows that they produce no such effect when applied to the nerves of the skin. In the same way, the rays of light applied to the nerves of the eye produce vision; but, no such effect takes place when they impinge upon the tongue.--Each of the organs of sense then possess a peculiar modification of nerves, which are excited by appropriate impressions.

By these organs we become acquainted with what passes around us; but the nervous system gives us notice of many changes which take place within our bodies. Internal pains point out to us the presence and situation of diseases; and the disagreeable sensations of hunger, thirst, and fatigue, incline us to give refreshment and repose to the body. It is also by means of the nervous system, that we experience the passions and emotions of the mind.

There are some animals so simple in their structure, that neither brain, nor organs of sense have been detected; yet they are endowed with motion, and are capable of selecting and swallowing their food, and expelling their excrement; and as these acts appear to be voluntary, we must conclude, that they possess nervous matter, though it be so interwoven with the rest of their structure that we cannot exhibit or detect it.

All these different structures which have been described as entering into the formation of a perfect animal, are soft and flexible in themselves, and, in order to the right performance of their functions, require the support of a substance of considerable firmness, which may preserve them in their relative situations, and give a general shape to the body. For this purpose, _bones_ are formed in the higher orders of animals. They consist of a certain portion of animal matter, on which their powers of life depend, mixed with a portion of earthy matter, which gives them a degree of solidity. The firmest substance in the body, composed entirely of animal matter, is cartilage, which possesses, however, too little solidity for the support of animals of considerable size, living in so rare a medium as air. Hence it happens that when the earthy part is, by disease, abstracted from the bones, they become bent and deformed by the weight of the body, or the action of its moving powers. In fishes, however, who inhabit a denser medium, cartilage becomes a convenient structure, being sufficiently firm for their support, and, from its lightness, better suited to their condition.

Had the osseous system been merely intended to give shape to animals, and preserve the relative position of their parts, it might, for any useful purpose, have been as well formed of one piece; and accordingly, when almost all the bones of the body have been anchylosed, or immoveably united to each other by disease, the functions of life have gone on uniformly to an advanced age. There is a remarkable skeleton of this kind preserved at Trinity College, Dublin; where all the large bones of the body are immoveably united together, except the lower jaw, and the joints of the fingers; every joint in the body was immoveable, and yet this person lived to an old age. In order, however, that animals may enjoy a power of changing their situation, the osseous system has been composed of a variety of pieces, and an apparatus added by which this may be easily effected. This is accomplished by adapting the ends of bones to each other so as to form joints, which vary in different parts of the body according to the motion of the part, some being formed for strength, others for extent and variety of motion; the two being incompatible, and never found in the same joint.

In the formation of a joint, however, it appears that two surfaces of bone would move with considerable attrition upon each other, not being capable of a sufficient degree of smoothness; it is therefore necessary, in order to diminish attrition, that a substance be interposed having a high degree of polish; this is supplied by cartilage, with which the ends of all bones, performing motion, are covered; and as animals, both from the common occurrences of life, and from accident, are liable to considerable shocks, in order to guard the system, as much as possible, against injury from these sources, cartilages are endowed with a considerable degree of elasticity, and thus by their reaction are capable of evading certain degrees of violence.

The smoothness of cartilage, however, only prevents attrition to a certain degree; that joints therefore may move with all possible ease and freedom, a fluid is interposed called _sinovia_. This is separated from the blood, by the vessels distributed to the inner surface of the joint, and is the most slippery of all fluids.

In order that bones may not be separated from each other, but preserve their relative situations, with a certain capacity of motion, it is requisite that they should be joined together; this is done by the ligaments surrounding a joint, which are of two kinds. The one adapted to the firm junction of the bones with each other, upon which the strength of the joint depends; the other loosely attached round the ends of contiguous bones, to secrete sinovia, and retain it in its proper situation; and hence called capsular or purse-like ligament.

This kind of structure, endowed with a power of secreting sinovia, is not confined to the joints alone; for in many parts of the body, where muscles during their action rub on bones, or tendon on tendon, small bags are formed for supplying sinovia, which are called bursæ mucosæ.

As all these parts subservient to motion are inert in themselves, that animals may enjoy the means of changing their situations and attitudes, a power must be applied to the bones for this purpose, which is supplied by muscular action. Thus we find the bones clothed with _muscles_, which give, in a great measure, the external shape to the body, and act in considerable numbers on the joints, particularly those which possess much motion.

All animals have a muscular structure entering into their composition, with some variety in its appearance. Muscles are generally fibrous to the eye, and in Man and Quadrupeds are of a red color; in some animals, however, these circumstances are not at all obvious. Thus in many fishes, the muscles are white, and put on a flaky appearance; whilst in the fresh water Polypus, which possesses a great degree of contractile power, no fibres can be seen. So that it is not necessary that these properties should be obvious in the muscles of all animals. Thus no person has ever seen the fibres in the muscles of a Flea, yet no animal can exert greater muscular power. In the same way, many parts of the body possess a contractile power, which have no apparent fibrous structure; the best example of this, is the skin of the scrotum. The redness of a muscle, in fact, depends in a great measure on the degree of exertion it undergoes; thus when a limb becomes motionless from palsy, the muscles uniformly become pale.--The function of a muscle consists in its contracting or shortening itself, in consequence of the application of certain stimuli or excitements; the effect of this contraction is, that the different bones to which the muscles are attached are moved in various directions. Thus (to give an example) a muscle affixed to two contiguous bones, by shortening itself, brings those points to which it is affixed nearer to each other; and, from this mechanism, arise all the motions of the body. The greatest part of the muscles which put the limbs in motion by their contractions, are said to act under the excitement of volition, or, in other words, are under the control and influence of the will, and are therefore called voluntary muscles. There are many muscles, however, which are not excited by volition, and are therefore called involuntary. As these are directed by influences, and perform the actions on which life immediately depends, they, for obvious reasons, are not only put beyond the powers of the will, but are enabled to carry on their contractions and motions without interruption or fatigue, entirely independent of its direction or our consciousness. In this manner the heart performs the circulation of the blood, and the stomach and intestines give the requisite motion to the food.--There are many other excitements which produce contraction in muscles, such as the passions and emotions of the mind, and various mechanical and chemical stimuli. Some of them occasionally excite the voluntary muscles of the body to a degree of action, over which volition has no control. Thus a person in an ordinary state of mind, can walk more or less quietly as suits his convenience; but it occasionally happens, we shall say in the field of battle, that the passion of fear is excited; this excitement frequently disregards the power of the will, and strongly excites the muscles employed in running away.[197]

In most animals, there is connected with the muscles another kind of structure called _tendon_, which consists in a white substance very different from muscles, but having a fibrous structure. Although tendons are not necessary to the action of muscles, yet there are several advantages derived from them; they occupy much less room than muscles, and can be placed in greater numbers around the joints, so as to preserve the beauty and uniformity of the limbs. They may be considered as living cords, joining the muscle to the bone on which it is to act, and, being more scantily supplied with blood than muscles, make a smaller quantity of blood necessary to the system, which is certainly a convenience. Although the different parts of the body vary very much in their functions and degree of motion; yet, it is convenient, that they should be all united together by a substance of considerable elasticity. This is done by the interposition of _cellular membrane_, which is the general connecting medium throughout the body, attaching each organ to its neighbor, but allowing sufficient play for the performance of its function.

It is in the cellular membrane of different parts of the body that fat is deposited; and from the seeming caprice of nature, in overloading some animals, and entirely denying it to others, its use has been thought inconsiderable in the system. When, however, we remark, that fat is taken up in some diseases where the appetite is impaired; and that torpid animals, before hibernation, have a large quantity of it accumulated, and come out of that state quite emaciated: and that bees, who have no fat in their bodies, lay up a stock of food, having the same chemical properties, against their hibernating season; it appears very probable, that one use of fat is to form a reservoir of nutriment, which supplies the wants of an animal when food is not introduced by the stomach.

If we add the _skin_ to the cellular membrane, we may say, without these the beauty and symmetry of the exterior would have been much diminished. We should have seen the raw muscles in all their actions, and the naked nerves exposed to the air and to injury. There would have existed deep fissures between the muscles, cavities in almost every part, and the body would have presented the sad appearance it now does in consumption.[198] But the cellular substance in some places only separates one part from another, or affords a slippery surface for one muscle to slide over the other: in others forming membranes or fascia to hide, to bind down and strengthen different organs; while in others admitting into its cells an oily substance, becomes fat, and fills up all the interstices, rounds off all prominences, softens acute lines, and gives a graceful softness and contour to the whole. And the skin enveloping in a close case, keeps all compact, and hides from the eye whatever might be offensive: while, at the same time the cutis or true skin serves for a surface for the nerves and exhalent vessels to terminate, the cuticle or scarf skin defends them from injury, and moderates their excessive sensibility.