236 NERVOUS SYSTEM the optic tubercles, and is generally small. The olfactory nerves are apparently continuous with the anterior or cerebral ganglia. The brain of fishes is composed of lobes placed one behind the other so as to form a sort of knotted cord. The optic tubercles, called also tulercula quadrigemina, and the cerebellum, are placed behind the cerebral ganglia, and are larger in proportion to the latter than in rep- tiles. The spinal cord differs essentially in the different classes of vertebrata only in the num- ber of pairs of nerves which arise from it, this number being always proportioned to that of the vertebrae. Functions of the several parts of the Nervous System. The first question that meets us in this inquiry is : What are the vital endowments of nerves and nervous centres? "When we expose and irritate a nerve by chem- ical, mechanical, or electrical stimulus, we ob- serve no visible change in the nerve itself, but we produce indications of pain and contrac- tion of the muscles to which the nerve is dis- tributed. We infer therefore from the effects of the irritation that it has produced some change in the nerve, or excited in it some vital endowment. This change has been compared to that wrought in a piece of malleable iron by the galvanic current ; and the analogy has suggested the term polarity or polar force, as applicable to the condition produced in nerves by the application of a stimulus. This property of nerves, by which they respond to external irritation, producing pain or muscular contrac- tion, is called irritability ; and one of its most remarkable characteristics is the instantane- ousness with which the effect follows the irri- tation. This irritability or nervous force, as it is also called, can be excited at the centres and extremities as well as in the course of the nerves. The effects of irritation vary consid- erably, however, in different parts of the ner- vous system. There are certain ganglia and nerves, the irritation of which produces effects that are entirely peculiar to themselves. Thus, irritation of the optic tubercles or nerves causes the sensation of light, irritation of the auditory nerve gives rise to the sensation of sound, and irritation of the olfactory and gustatory nerves to the sensations of smell and taste. This dif- ference in the effects produced by the irri- tation of different nerves is the basis of the division of the nerves into : 1, motor nerves, or those in which irritation produces muscular contraction; 2, sensitive nerves, or nerves of common sensibility, in which irritation is fol- lowed by an agreeable or painful feeling, ac- cording to the nature or degree of the stimu- lation ; and 3, nerves of special sense, in which irritation excites the peculiar sensations of light, sound, taste, &c. Many sensitive nerves arise from nervous centres in such close prox- imity to motor nerves, that a stimulus applied to the former will react upon the latter, and produce not only a direct sensation, but what is technically called a reflex action. This property of reflection was supposed by Mar- shall Hall to reside in special nerve's which possessed both an incident and a reflected fibre, or, as they are called, an afferent sensi- tive nerve leading to the centre, and an effer- ent motor nerve coming from the centre; these nerves he styled excito-motory. Prob- ably the majority of all the nerves in the body are compound in their constitution ; i. e., they are formed of sensitive and motor filaments combined in the same sheath, or fibrous cover- ing, called the neurilemma. There is no ap- preciable difference to the eye in the structure of sensitive and motor nerves. The difference in their endowments can only be ascribed, in the present state of our knowledge, to the dif- ferent nature of their central or peripheral connections. It is a remarkable fact that whatever part of a sensitive nerve be irritated, whether it be the centre, the middle, or the extremity, the same sensation will be pro- duced. There are many familiar illustrations of this fact. For years after the amputation of limbs, patients will experience the sensa- tion of fingers and toes; and when the trunk of a nerve is irritated, they will complain of pains or tingling in the extremities. The same law applies to nerves of special sense. Ordinarily their peculiar irritability is excited by stimuli applied to the periphery or ultimate distribution of the fibres ; but occasionally, in consequence of disease, we have these special sensations caused by stimulation applied at the centre or origin of the nerves. Thus, dis- turbance of the circulation in the brain, or the existence of tumors or other local disease in the neighborhood of the centres of the nerves of special sense, gives rise to optical illusions, singular noises, disagreeable odors, &c. The stimuli that excite the nervous force or irritability are of two kinds, physical and mental. Physical stimuli embrace all exter- nal excitants of whatever nature light, heat, sound, odor, and every variety of chemical, mechanical, and galvanic irritant. Mental stimuli result from the exercise of the will and thought. In all voluntary movements the mind is the direct stimulus of the motor nerve ; and in certain deranged states of the nervous system, as in hysteria, the mind also seems to affect directly the nerves of sensation so as to excite pain. Motor nerves are never directly excited by physical stimuli. The lat- ter act always on motor nerves through the medium of sensitive nerves, and, as before stated, the actions so produced are termed reflex actions. The most remarkable of the physical stimuli, and the one most analogous in its effects to the stimulus of the will, is galvanism. The effect of this stimulus is mus- cular contraction, and there are certain pe- culiarities in its mode of action that are wor- thy of mention. 1. If the current be made to cross a nerve at right angles to it, no effect is produced ; but if it be made to pass along the fibre, for the smallest fraction of an inch, it will excite more or less violent con-
