720H 



PHYSIOLOGY OF THE NERVOUS SYSTEM. 



change rapidly propagated along the nerve, 

 from molecule to molecule, from the point of 

 application of the stimulus. The change is 

 obviously analogous to that which takes place 

 in the particles of a piece of soft iron, in virtue 

 of which the iron acquires the properties of a 

 magnet, so long as it is maintained in a certain 

 relation to a galvanic current ; the magnetic 

 power being instantly communicated when the 

 circuit is completed, and as rapidly removed 

 when it is interrupted. 



The action of the stimulus, then, excites a 

 state of polarity of the particles of the nerve 

 stimulated ; and this polar state may be in- 

 duced in other particles, whether muscular or 

 nervous, with which the nerve stimulated may 

 be in organic connexion. Just as the polar 

 state of the electrical apparatus is capable of 

 being communicated to the piece of soft iron, 

 which thereby acquires the well-known mag- 

 netic properties during the continuance of the 

 excited polarity. 



Thus, then, we learn that such is the nature 

 of the nerve fibre, that under the application 

 of a stimulus, mechanical, chemical, or galvanic, 

 it is capable of generating a polar force analo- 

 gous in many particulars to that of muscle; 

 this force we call the nervous force, v is nervosa, 

 or nervous polarity* 



And if we examine the ordinary mode of the 

 development of the nervous force, in the usual 

 actions of the frame, we find that under the 

 influence of a mental stimulus, the will, it is 

 propagated from the nervous centre along the 

 nerves to muscles, or under the influence of a 

 physical stimulus it is propagated along the 

 nerves to the centres, where it is capable of 

 exciting either a sensation or muscular motion 

 in a secondary manner, or both. 



But the application of a physical stimulus 

 to a nervous centre may cause the development 

 of nervous force, which may be conducted 

 away from it by nerves which are implanted in 

 it. And thus we learn that the same polar 

 condition which may be produced in nerves is 

 equally capable of being excited in nervous 

 centres. The polar condition of the nerve fibre 

 may be propagated to the nervous centre, or that 

 of the nervous centre to the nerve fibre. 



In some of the nervous centres, however, no 

 visible change of any kind takes place upon the 

 irritation of the nervous matter, nor does the ani- 

 mal seem to suffer pain. Such is the case when 

 the hemispheres of the brain are the subject of 

 experiment. We are not to infer from this that 

 the nervous force is not developed in these 

 centres, but that they have no direct connexion 

 with the muscular system, nor have they that 

 peculiar organization which would enable them 

 when irritated to excite painful sensations. 



There are certain nerves which when stimu- 

 lated excite neither muscular motion nor cora- 



* I have been in the habit of taking this view of 

 the nervous force in my lectures for the last four or 

 five years, and of using the term, nervous polarity, 

 as expressive of the nature of the nervous force. 

 This term has likewise been adopted by Mr. Bow- 

 man and myself in our work on the Physiological 

 Anatomy and Physiology of Man, vol. i. p. 56, and 

 in the chapters on the Nervous System, passim. 



mon sensation or pain, but a sensation peculiar 

 to themselves. Thus if the optic nerve be sti- 

 mulated by a mechanical or galvanic stimulus, 

 a sensation of light is produced ; if the auditory 

 nerve be stimulated in like manner, a sensation 

 of sound is produced. 



These facts prove not only that a peculiar 

 force is generated by the nervous matter, but 

 they also show that the nerve fibres in the cen- 

 tres, as well as in the nerves, possess special 

 endowments depending, in all probability, upon 

 their central as well as upon their peripheral 

 connexions. Thus nerve-fibres connected with 

 muscles are capable of exciting muscular con- 

 traction, and are therefore called motor or mus- 

 cular nerves. Nerve-fibres, which are distri- 

 buted to a sentient surface, as the skin or mu- 

 cous membrane, and have a certain relation 

 with that part of the nervous centre which con- 

 stitutes the centre of sensation, (vide p. 711,) 

 are when stimulated capable of exciting a feel- 

 ing which may be agreeable or painful, accord- 

 ing to the degree of stimulation. These are 

 called sensitive nerves, or nerves of common 

 sensation. To the class of sensitive., nerves 

 belong those which, owing no doubt to a pecu- 

 liarity in their connexion with the centre, as 

 well as to their relation to a special apparatus 

 at their periphery, develope peculiar sensations, 

 as the nerves of sight, hearing, taste, &c., and 

 they have been distinguished as nerves oj' special 

 sensation. 



Very many sentient nerves are implanted in 

 the nervous centre near to certain motor nerves, 

 so that a stimulus applied to the former is 

 capable of reacting upon the latter, and of ex- 

 citing motion through their connexion with the 

 muscles. Dr. M. Hall, however, ingeniously 

 supposes that this power resides only in a par- 

 ticular class of nerve-fibres (and not in the 

 ordinary sentient nerves through their closeness 

 of relation with the ordinary motor nerves). A 

 nerve of this kind would constitute an arc, 

 consisting of an incident and a reflex portion, 

 which are united at the nervous centre. The 

 stimulus is conveyed to the centre by the inci- 

 dent portion, and is then reflected into the 

 reflex or motor portion. Such nerves, Dr. Hall 

 designates excito-motor. We shall examine 

 further on the grounds of this hypothesis. 



It is an important fact, which Sir C. Bell 

 was the first clearly to prove, that nerve-fibres 

 of different endowments may be bound together 

 in one sheath, forming, in anatomical language, 

 one nerve. Thus a nerve may contain sentient 

 and motor fibres, as the median nerve in the 

 arm, or if we admitted Dr. Hall's hypothesis, 

 it might contain sentient, motor, and excito- 

 motor fibres. And most nerves in the diffe- 

 rent regions of the body are of this description, 

 i. e. compound nerves, made up of sentient and 

 motor fibres bound together in the same sheath, 

 in very different proportions. In many of these 

 nerves, as in the spinal nerves, and the fifth 

 pair, the separation of the fibres of motion from 

 those of sensation exists at the implantation in 

 the centre, and there the fibres of each endow- 

 ment are collected into a separate bundle, 

 which possesses the endowment proper to its 

 constituent fibres. These are the roots of these 



