BOOK III. 



THE CENTRAL NERVOUS SYSTEM AND ITS INSTRUMENTS, 



CHAPTER I. 



THE SPINAL CORD. 



ON SOME FEATURES OF THE SPINAL NERVES. 



471. WE have called the muscular and nervous tissues the master 

 tissues of the body ; but a special part of the nervous system, that which we 

 know as the central nervous system, the brain and spinal cord, is su- 

 preme among the nervous tissues and is master of the skeletal muscles as 

 well as of the rest of the body. We have already (Book I., Chapter III.) 

 touched on some of the general features of the nervous system, and have 

 now to study in detail the working of the brain and spinal cord. We have 

 to inquire what we know concerning the laws which regulate the discharge 

 of efferent impulses from the brain or from the cord, and to learn how that 

 discharge is determined, on the one hand, by intrinsic changes originating, 

 apparently, in the substance of the brain or of the cord, and, on the other 

 hand, by the nature and amount of the afferent impulses which reach them 

 along afferent nerves. 



As we shall see, the study of the spinal cord cannot be wholly separated 

 from that of the brain, the two being very closely related. Nevertheless, it 

 will be of advantage to deal with the spinal cord by itself as far as we can. 

 The medulla oblongata or spinal bulb 1 we shall consider as part of the brain. 

 But before we speak of the spinal cord itself, it will be desirable to say a 

 few words concerning the spinal nerves, that is to say, the nerves which 

 issue from the spinal cord. 



We have already seen ( 92) that each of the spinal nerves arises by two 

 roots, an anterior root attached to the ventral or anterior surface, and a pos- 

 terior root attached to the dorsal or posterior surface of the cord. We have 

 further seen that the latter bears a ganglion, a " ganglion of the posterior 

 root" or "spinal ganglion," as we have ( 93) studied the structure of this 

 ganglion. 



We stated at the same time that while the trunk of a spinal nerve con- 

 tained both efferent and afferent fibres, the efferent fibres were gathered up 

 into the anterior root and the afferent fibres into the posterior root ; but we 

 gave no proof of this statement. 



472. Before we proceed to do so, it will be as well to say a few words 

 on the terms " efferent " and " afferent." By efferent nerve-fibres we mean 

 nerve-fibres which in the body usually carry impulses from the central ner- 



1 The term medulla oblongata is not only long, but presents difficulties, since the word 

 medulla is now rarely used to denote the whole spinal cord (medulla spinalis) but is gen- 

 erally used to denote the peculiar coat of a nerve-fibre, the white substance of Schwann. 

 In using instead the word bulb or, if necessary, spinal bulb there is little fear of confu- 

 sion with any other kind of bulb. The adjective is in not uncommon use, in such phrases 

 as " bulbar paralysis." 



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