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.
In the highest class of animal life the brain is at once the seat of intelligence and sensibility, and the origin of the nervous system. In the lower animals intelligence and sensibility decrease exactly in proportion to the deviation of their nervous system from this high standard. The forms of the nervous system are more and more degraded as the animals sink in the scale of being, till at last creatures are found in which nerves have only been discovered with the microscope; others apparently have none, consequently they have little or no sensibility.
The brain and the spinal cord enclosed in the vertebræ of the backbone form a nervous system, which in the vertebrated creation is equal to all the contingencies and powers of these animated beings, but is beyond all comparison most perfect in the human race. The brain alone is the seat of consciousness, for the spinal cord, though intimately connected with it, and of a similar ‘mysterious albuminous electric pulp,’ appears to have no relation to the faculties of perception and thought, yet it is essential to the continuance of life. It is a distinct nervous centre which generates muscular energy in man and animals corresponding to external impressions, but without sensation, and is entirely independent of the will; the vegetative functions of respiration, the contractions of the heart, circulation of the blood, and digestion, are carried on under every circumstance, even during sleep. The reason of their being independent of sensation and the will is, that the nerves in the organs performing these functions never reach the brain, which is the seat of intelligence and sensation, but they form what is called the reflex system; for any impressions made upon them are carried to the upper part of the spinal cord alone, and are reflected back again to the muscles of the heart, lungs, &c., which, by their contractions, produce these involuntary motions. For instance, the flow of blood into the cavities of the heart while dilating, acts upon the nerves, and these excite a rhythmical movement in the muscular fibres of the heart. For there is a vital contractility in muscular tissue which is one of the most universal attributes of living beings, and is probably the sole cause of motion in the lowest grades of life, and the movements produced by it in the higher grades are in all cases the most directly connected with the vegetative functions. The involuntary reflex system of nerves constitutes the chief locomotive power in a number of the lower animals; but it forms a continually decreasing portion of the whole nervous system in proportion as animals rise in the scale of life, till in man its very existence has been overlooked. If the spinal cord were destroyed, instant death would be the consequence; whereas infants born without brain have sucked and lived for a day or two.