CHAPTER III. 



ON THE MOKE GENEKAL FEATURES OF NEKVOUS 



TISSUES. 



88. IN the preceding chapter we have dealt with the pro- 

 perties of nerves going to muscles, the nerves which we called 

 motor, and have incidentally spoken of other nerves which we called 

 sensory. Both these kinds of nerves are connected with the brain 

 and spinal cord and form part of the General Nervous System. 

 We shall have to study hereafter in detail the brain and spinal cord ; 

 but the nervous system intervenes so repeatedly in the processes 

 carried out by other tissues that it will be desirable, before pro- 

 ceeding further, to discuss some of its more general features. 



The Nervous System consists (1) of the Brain and Spinal Cord 

 forming together the cerebrospinal axis, or central nervous system ; 

 (2) of the nerves passing from that axis to nearly all parts of the 

 body, those which are connected with the spinal cord being called 

 spinal, and those which are connected with the brain, within the 

 cranium, being called cranial ; and (3) of ganglia distributed along 

 the nerves in various parts of the body. 



The spinal cord obviously consists of a number of segments or 

 metameres, following in succession along its axis, each metamere 

 giving off on each side a pair of spinal nerves ; and a similar 

 division into metameres may be traced in the brain, though less 

 distinctly, since the cranial nerves are arranged in manner some- 

 what different from that of the spinal nerves. We may take a 

 single spinal metamere, represented diagrammatically in Fig. 24, 

 as illustrating the general features of the nervous system ; and 

 since the half on one side of the median line resembles the half 

 on the other side, we may deal with one lateral half only. 



Each spinal nerve arises by two roots. The metamere of the 

 central nervous system C consists, as we shall hereafter see, of grey 



