Although we are ignorant of the nature of the cause which regulates the uniform performance of this series of phenomena, thus more or less extensively displayed in the economy of different animals, we are nevertheless convinced that such a cause must exist, and are hence naturally led to distinguish the phenomena by some appropriate term. Thus observing that the human body, and the bodies of animals which bear it resemblance, possess locomotive powers, can regulate their actions, and are capable not only of resisting the laws which govern inanimate substances, but are enabled to act upon these substances in direct opposition to these laws, we employ the terms life, vitality, and vital power, to express the phenomena which thus distinguish animate from inanimate matter; and in order simply to determine the import of those terms, we may take a general view of those powers which a living animal body possesses, and which cease with its existence.
When we compare the living with the dead body, the most striking circumstance we observe is, that the former was surrounded by the same chemical agents which are capable of producing the decomposition and destruction of its soft parts after death; hence it becomes evident, that its component elements must have been sustained and preserved by some superior power, which ceases to act at the moment of its dissolution.
Of all the phenomena which enter into the general idea of life, this power of self-preservation, or the capability of resisting the laws which govern inanimate matter, appears the most essential. Without this principle we can form no conception of life, since it evidently exists without interruption till the moment of dissolution. It is this principle which, communicated to an egg, enables it to resist for a certain period the powers of heat, cold, and putrefaction; a principle of which the addle or barren egg is entirely devoid. Thus we find from Mr. J. Hunter’s experiments, that an impregnated egg is longer in freezing than an addle egg, and every one knows that the former remains sweet or free from putrefaction much longer. This principle, which we may consider the most simple state of existence, is limited in its duration; and for its maintenance, the performance of no active function is necessary. Hence it continues in the egg either quiescent for a certain time, and is gradually destroyed; or, by the agency or stimulus of heat, it acquires the accession of the power of action, which assimilates inanimate matter into a living form, and, at length, exhibits in the chick all the phenomena of a more perfect state of existence, which may be distinguished by the term active life. In this state, many other phenomena of vitality are exhibited. Besides the power of self-preservation, an internal principle of support and reparation, and the power of performing the important actions of circulation, respiration, digestion, &c., which are subservient to this principle, is given to animals. These form the features of what we call life, as it appears in man, and the higher orders of animals, and certainly constitute its most useful, though not most essential part. For how little superior is an egg, or a torpid animal, to vegetable or inanimate matter, till the former contain a living chick, the latter become an active animal? Thus, though life may subsist under the quiescent form of self-preservation, it requires the accession of certain principles, and a power of performing various important actions, to display its chief characters. The economy then of an egg, and of a perfect animal, such as man, may be considered as examples of the most simple and extensive phenomena of vitality. These, however, are more or less perfectly exhibited in the different orders of animals. It belongs, for instance, to the economy of certain animals, which at one time of the year perform active functions, to become torpid at the approach of winter. In these creatures, respiration, digestion, and every function which characterizes active life, is suspended; as in the egg, the principle of self-preservation, that latent spark of vitality, alone remains, by which we distinguish torpidity from death. This condition, however, is not of long continuance; at the approach of summer’s warmth, the power of action is again called forth, active functions are superadded to the principle of self-preservation, and life, before quiescent and obscure, now resumes its most perfect form; or, in other words, the animal just now inert and motionless, respires again; its heart beats, its blood circulates, its muscles resume their accustomed motions, and it leaves its winter quarters in search of food. Having now assumed the nature of an active animal, the performance of the functions characteristic of that state (which we shall now proceed to describe) becomes requisite; and, first, the circulation of a fluid which we call blood.[189]
This fluid differs in its appearance in the different orders of animals, though, in its essential properties there is little variety; the appearance of the blood in man, and the more perfect animals, is that of a red fluid, having a certain degree of viscidity, not being limpid like common water. Though it appears to be a homogenous fluid whilst circulating, or at the moment it escapes from its vessels, it is composed of three parts, essentially differing from each other; of particles, upon which the color of the blood depends; of coagulating lymph, which has the property of becoming spontaneously solid under certain circumstances, and from which various structures in the body are formed; and of a limpid fluid called serum, which dilutes the coagulating lymph, and fits it for circulating through blood vessels of a very minute size. In some of the lower orders of animals, however, the color of the particles of the blood is green, in others white, corresponding with the color of the animal; in others there is no color whatever in the blood, so that it is either void of particles, or they are transparent, so as not to be seen. But this deficiency must be considered as making no great difference in the blood itself, as its particles do not appear to be its most essential part. Many microscopical observations have been made to determine the size of a single particle of the blood, and according to the observations of several philosophers, the diameter of a single particle in man has been computed at the 3,000th part of an inch. The size of the particles in red blooded animals, is found not to correspond with the size of the animal. They are as large in the mouse as the elephant, larger in some insects than in man, smaller in the ox. They are in prodigious numbers, so as to give color to the blood, and of all its parts appear to be renewed the most slowly; thus when animals are frequently bled, the flesh becomes paler and paler.[190]
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.