The waste of the tissues, and the aëration of the vital juices, that is, the exchange of the respiratory gases, are common to all animals. The heart, upon whose expansions and contractions the circulation of the blood depends, is represented in the lower animals by propelling organs of a variety of forms; and the organs of respiration differ exceedingly, according to the medium in which the animals live. Water, both fresh and salt, though a suffocating element to land animals, contains a great deal of air, not only in the state of gas, but also in solution, the quantity in solution being directly as the pressure; so that animals living in the deepest recesses of the ocean breathe as freely as those that live on land, but with respiratory organs of a very different structure. In the lowest classes, which have no respiratory organs at all, the gases are exchanged through their thin delicate skins.
The mechanical forces act within the living being according to the same laws as they do in the external world: the chemical powers too, which are the cause of digestion, heat, and respiration, follow the same laws of definite and quantitative proportion as they do in inert matter; but neither the mechanical forces, nor the physical powers, could create a germ; nor could they even awaken its dormant state to living energy, unless a vital power existed in it, the origin of which is beyond the reach of man.
Animals are endowed with nerve-force, in addition to mechanical force and the physical powers which are common to them and vegetables; a force which constitutes their prime distinction, which is superior to all the other powers from its immediate connection with mind, and which becomes more evident, and more evidently under the control of the animal, in proportion as the animal approaches the higher grades of life, and only attains its perfect development in the human race.
The bones of man and the higher animals are clothed with a system of muscles, so attached that the head, eyes, limbs, &c., can be moved in various directions. In each of these muscles the fibres of two sets of nerves ramify, namely, the sensory and the motor nerves.
The sensory nerves convey external impressions to the brain, and by them alone the mind is rendered conscious of external objects. The impressions made by light and sound upon the eye and the ear, or by mechanical touch on the body, are conveyed by the sensory nerves to the brain, where they are perceived, though the impressions take place at a distance from it. Conversely, the mind or will acts through the brain on the motor nerves, which by alternately contracting, relaxing, and directing the muscles, produces muscular motion. Thus the motor nerves convey the emotions of the mind to the external world, and the sensory nerves convey the impressions made by the external world to the mind. By these admirable discoveries, Sir Charles Bell has proved that ‘we are placed between two worlds, the invisible and the material;’ our nervous system is the bond of connection. The connection, however, between the mind and the brain is unknown: it has never been explained, and is probably inexplicable; yet it is evident that the mind or will, though immaterial, manifests itself by acting on matter; that is, as a power which stimulates the nerves, the nerve-force acting on the muscles. Mental excitement calls forth the most powerful muscular strength, and an iron will can resist the greatest nervous excitement. The nervous and muscular forces are perpetually called into action, because, for distinct perception, the muscles require to be adjusted. Mind is passive as well as active: we may see an object without perceiving it, and we may hear a sound without attending to it. We must look in order to see, listen in order to hear, and handle in order to feel; that is, we must adjust the muscular apparatus of all our senses, of our eyes, ears, &c., if we would have a distinct perception of external exciting objects: and that is accomplished by the power of mind acting upon matter.
Dr. Carpenter has shown that it is by a series of forces acting upon matter that man conveys his ideas to man, the sonorous undulations of the atmosphere being the medium between the two. On one side the will, or power of mind, acts upon the nerves, nerve-force acts upon the muscles of speech, and these muscles, while in the act of speaking, produce sonorous undulations in the atmosphere. On the other side, these undulations are communicated by the mechanism of the ear to the auditory nerves, exciting nerve-force, and nerve-force acts upon the mind of the hearer. ‘Thus the consciousness of the speaker acts upon the consciousness of the hearer by a well-connected series of powers.’
Nerve-force generates, directly or indirectly, light, heat, chemical power, and electricity. When the optic nerve is pressed in the dark, a luminous ring is seen round the eye, and a blow on the face excites a flash of light. Nervous excitement, by accelerating respiration, increases the chemical combination of the oxygen of the air with the carbon of the blood, and thus produces animal heat. But the development of electricity by nervous and muscular force, is one of the most unexpected and singular results of physiological research.
MM. Matteucci and Du Bois Reymond have proved that the intensity of the nervous and muscular forces is at a maximum when the muscles are contracted; and that if each arm of a man be put in contact with a wire of a galvanometer so as to form an electric circuit, an instantaneous deviation of the needle will take place, now in one direction and now in the other, according as he contracts his right arm or his left. The electricity thus evolved, when conveyed to the needle through several miles’ length of coiled insulated wire, will cause a deflection amounting to sixty or seventy degrees, according to the strength of the man—that is, according to his muscular and nervous force; the amount of the electricity being exactly in proportion to the amount of muscular force.
It appears that the electric currents in the nerves are eight or ten times stronger than those in the muscles. M. Helmholtz found that the time required to contract a muscle, together with the time required to relax it again, is not more than the third of a second, and is a constant quantity, for the compensation of energy prevails also in organic nature. He also found that the motion or velocity of the electric current in a man is at the rate of 200 feet in a second. The electric equivalent, as determined by M. Helmholtz, is equal to the electricity produced in a voltaic battery by the seven millionth part of a milligramme of zinc consumed in the ten-thousandth part of a second, a milligramme being the 0·015432 part of a grain.
The contraction and muscular action or mechanical labour produced by the passage of an electric current through a nerve is 27,000 times greater than the mechanical labour which results from the heat disengaged by the oxidation of that small quantity of zinc requisite to generate the electricity; that is to say, the mechanical labour really produced by the contraction of the muscles is enormously greater than the labour corresponding to the zinc oxidized. In fact, the electric excitement of a nerve is analogous to an incandescent particle or electric spark that sets fire to a great mass of gunpowder. This result, and the association between the greatest activity of respiration and the intensity of the muscular energy, led M. Matteucci to suspect that a chemical action must take place in the interior of a muscle during its contraction; and he found by experiment that there actually is what he calls a muscular respiration, namely, that the muscles themselves absorb oxygen, and give out carbonic acid gas and nitrogen when contracted. This kind of respiration is more or less common to all animals; if impeded, the blood is imperfectly oxygenized, and loss of animal heat is the consequence. The heat that is perpetually escaping from animals is replaced, by the combustion of the carbon of the tissues or of the food with the oxygen inhaled by the lungs and the skin.