It must be evident to every reader that but very few of the striking features of animal life have been mentioned in the rapid survey which has been taken of the progress of animal organization. The subject is so extensive that it would be quite impossible to embrace it within any reasonable limits; and it furnishes matter so curious and so instructive, that, having once entered on it, it would have been difficult to have made any selection, and we must have devoted a volume to the æsthetics of natural science. Passing it by, therefore, with the mere outline which has been given, we must proceed to consider some of the conditions of vitality.
Bell has proved that one set of nerves is employed in conveying sensation to the brain, and another set in transferring the desires of the will to the muscles. By the separation of a main branch of one of the nerves of sensation, although all the operations of life will still proceed, the organ to which that nerve goes is dead to its particular sense. In like manner, if one of the nerves of volition is divided, the member will not obey the inclination of the brain. It is evident, therefore, although many of the great phenomena of vital force are dependent on the nervous system, and the paralysis of a member ensues upon the separation or the disease of a nerve, that the nerves are but the channels through which certain influences are carried. The vis vitæ or vital principle—for we are compelled by the imperfection of our knowledge to associate under this one term the ultimate causes of many of the phenomena of life—is a power which, although constantly employed, has the capability of continually renewing itself by some inexplicable connection existing between it and many external influences. We know that certain conditions are necessary to the health of animals. Diseased digestion, or any interruption in the circulation of the blood, destroys the vital force, and death ensues. The processes of digestion and of the circulation are perfectly understood, yet we are no nearer the great secret of the living principle.
Animals are dependent on several external agents for the support of existence. The oxygen of the air is necessary for respiration. Animal heat, as will be shown presently, is in a great measure dependent upon it. The external heat is so regulated that animal existence is comfortably supported. Electricity is without doubt an essential element in the living processes; and, indeed, many physiologists have been inclined to refer vital force to the development of electricity by chemical action in the brain. This view has, however, no foundation in experiment beyond that afforded by the appearance of electric currents, when the brain is excited. This proves no more than that the operations of mind develope physical power in the matter with which it is mysteriously connected.
The phenomena of the Torpedo and Gymnotus we have already noticed,[264] and there are other creatures which certainly possess the power of secreting and discharging electricity. Galvani’s experiments, and those of Aldini, appear to show—and the more delicate researches of Matteucci have satisfactorily determined—that currents of electricity are always circulating in the animal frame;—that positive electricity is constantly passing from the interior to the exterior of a muscle. Matteucci, by arranging a series of muscles, has formed an electric pile of some energy.[265] These currents have been detected in man, in pigeons, fowls, eels, and frogs.
In the human body it is evident a large quantity of electricity exists in a state of equilibrium. Du Bois Raymond has shown that we may by mere muscular motion give rise to electric currents which can be measured by the galvanometer. This, however, may be said of every substance. It is perhaps more easily disturbed in the human system; indeed, the manifestation of sparks from the hair and other parts of the body by friction is not uncommon. Every chemical action, it has been already shown, gives rise to electrical manifestations; and the animal body is a laboratory, beautifully fitted with apparatus, in which nearly every chemical process is going on. It has been proved that acid and alkaline principles are constantly acting upon each other through the tissues of the animal frame; and we have the curious phenomena of endosmose and exosmose in constant effort, and catalysis or surface force, operating in a mysterious manner.[266]
With the refined physiological questions connected with the phenomena of sensation we cannot deal, nor will any argument be adduced for or against the hypothesis which would refer these phenomena to some extraordinary development of electric force in the brain. The entire subject appears to stand beyond the true limits of science, and every attempt to pass it is invariably found to lead to a confused mysticism, in which the real and the ideal are strangely confounded. Science stops short of the phenomena of vital action.
We cannot, however, but refer to the idea entertained by many that the brain is an electric battery, and the nerves a system of conductors. On this view Sir John Herschel remarks:—“If the brain be an electric pile constantly in action, it may be conceived to discharge itself at regular intervals, when the tension of the electricity reaches a certain point, along the nerves which communicate with the heart, and thus excite the pulsation of that organ.” Priestley, however, appears to have been the first to promulgate this idea.
Light is an essential element in producing the grand phenomenon of life, though its action is ill understood. Where there is light, there is life, and any deprivation of this principle is rapidly followed by disease of the animal frame, and the destruction of the mental faculties. We have proof of this in the squalor of those whose necessities compel them to labour in places to which the blessings of sunshine never penetrate, as in our coal-mines, where men having everything necessary for health, except light, exhibit a singularly unhealthy appearance. The state of fatuity and wretchedness to which those individuals have been reduced who have been subjected for years to incarceration in dark dungeons, may be referred to the same deprivation. Again, in the peculiar aspect of those people who inhabit different regions of the earth under varying influences of light, we see evidence of the powerful effects of solar action. Other forces, as yet undiscovered, may, in all probability do, exert decided influences on the animal economy; but, although we recognize many effects which we cannot refer to any known causes, we are perfectly unable to imagine the sources from which they spring.
It will be interesting now to examine the phenomena of animal heat, the consideration of which naturally leads us to consider the digestive system, the circulatory processes, and the effects of nervous excitation.
The theory, which attributes animal heat to the combination of the carbon of the food taken into the stomach with the oxygen of the air inspired through the lungs, has become a very favourite one. It must, however, be remembered that it is by no means new. The doctrines of Brown, known as the Brunonian system, and set forth in his Elementa Medicinæ, are founded upon similar hasty generalizations. Although, without doubt, true in a certain degree, it is not so to the extent to which its advocates would have us believe. That the carbonaceous matter received into the stomach, after having undergone the process of digestion, enters into combination with the oxygen breathed through the lungs or absorbed by the skin, and is given off from the body in the form of carbonic acid, and that, during the combination, heat is produced, by a process similar to that of ordinary combustion, is an established fact; but the idea of referring animal heat entirely to this chemical source, when there are other well-known causes producing calorific effects, is an example of the errors into which an ingenious mind may be led, when eagerly seeking to establish a favourite hypothesis.