From the observations which have been made in an earlier part of this article on the functions of individual nerves, it is sufficiently obvious that it is through the instrumen• tality of the nervous system that the mind influences the bodily organs, as when volition or emotion excites them to action; and that, conversely, impressions made on the organs of the body affect the mind, and excite mental perceptions through the same channel. "Lt this way," to quote the words of Dr. Todd, "the nervous system becomes the main agent of what has been called the life of relation; for without some channel for the trans mission of the mandates of the will to the organs of motion, or some provision for the reception of those impressions which external objects are capable of exciting, the mind, thus completely isolated, could hold no communion with the external world." The nature of the connection between the mind and nervous matter is, and must ever be the deepest mystery in physiology, and one into which the human intellect can never hope to penetrate. There are, however, many actions of the body in the production of which the mind has no share. Of this kind are the nervous actions, which are associated with the functions of organic life, such as digestion, respiration, and circulation. Again, there is another class of actions for which two nerves (an afferent or excitor, and a motor) and a nervous center are necessary. These are the actions known as reflex or excito motory, for the full investigation of which physiology is especially indebted to the labors of the late Dr. Marshall Hall. For example, the movement of the oesophagus in pro pelling the food onwards to the stomach, is caused by the stimulus of the food acting on the excitor or afferent nerves, which, through the spinal cord, excites the motor or afferent nerves, and thus give rise to the necessary muscular action. When the edge of the eyelid is touched, the excitor nerve (a branch of the ophthalmic division of Hie fifth or trifacial nerve) conveys the impression of the stimulus to the nervous center, and the eye is at once closed by the motor influence, which is transmitted by a branch of the facial nerve to the orbicular muscle. In such cases as these—and they form a very numerous class—the mind takes no part. In some of them it is conscious of the appli cation of the stimulus, as well as of the muscular act which follows; but even in these cases no effort of the will could modify or interrupt the sequence of the phenomena.
It has been already shown that the stimuli, by which the action of nerves is commonly excited, are of two kinds, mental and physical, and the change which these stimuli pro duce in a nerve develops the power known to physiologists as the 7.* nervosa. or nervous force. " The nervous force," says Dr. SI:arpey, in his Address on Physiology in 1862, "has long been likened to electricity, but rather through a vague perception of analogy than from any rigorous comparison. It is true that electric vague is developed in the nerves, and even exhibits modifications connected with different conditions of nervous action. Still, it must be borne in mind that the evolution of electricity is a common accompaniment of various processes involving chemical change, whether within the living body or in external nature: and the tendency of recent speculation is not towards the identification of the nerve force with electricity, but rather to suggest that the two stand related in the same way as electricity and other physical forces are related to each other—that is, as manifestations of a common force or energy, of which they, severally, are the special modifications." The velocity with impressions are transmitted by the nerves has been recently made the subject of investigation, but it is doubtful how far the observations are to he depended on, in consequence of the various sources of fallacy by which such experiments are beset. According to Hirsch, the velocity is 24 meters, or about 112 ft. per second in man; while Helmholtz fixes it at 190 ft. per second in the frog.
The description of the nervous system given in the foregoing pages is applicable, with slight modifications, to all the vertebrates; the main differences being in the degree of the development of the brain—a point which has been already noticed at the commence ment of the article BRAIN. For a sufficient notice of the plan of the nervous system in the invertebrate animals, the reader is referred to the articles ARTICULATED ANIMALS, Moar.rsc.t, and RADIAT.. It is only in the lowest subdivision of the animal kingdom, the PnoTozox, that no traces of a nervous system can be detected.
For further information on the subject of this article, the reader is referred to Dr. Carpenter's works on human mind Comparative ihysiology, to Dr. Todd's article on "The Nervous System" in fi:ie Cyclopedia of Anatomy and Physiology, to Todd and Bowman's Physiological Anatomy and Physiology of Man, and to Funke's Lehrbach der P4siologre