122 GENERAL FEATURES OF NERVOUS TISSUES. 



of the substance which can only execute amoeboid movements as being con- 

 tractile ? 



We may, if we admit that contractility is at bottom simply the power of 

 shifting the relative position of particles, admit that muscular contraction is 

 a specialized form of contraction. In a plain muscular fibre (which we may 

 take as simpler than the striated muscle) the shifting of particles is special- 

 ized in the sense that it has always a definite relation to the long axis of the 

 fibre; when the fibre contracts a certain number of particles assume a new 

 position by moving at right angles to the long axis of the fibre, and the fibre 

 in consequence becomes shorter and broader. In a white blood-corpuscle, 

 amoeba, or other organism executing amoeboid movements, the shifting of the 

 particles is not limited to any axis of the body of the organism ; at the same 

 moment one particle or one set of particles may be moving in one direction, 

 and another particle or another set of particles in another direction. A 

 pseudopodium short and broad, or long, thin, and filamentous, may be thrust 

 out from any part of the surface of the body and in any direction ; and a 

 previously existing pseudopodium may be shortened, or be wholly drawn 

 back into the substance of the body. 



CHAPTER III. 



ON THE MORE GENERAL FEATURES OF NERVOUS TISSUES. 



92. In the preceding chapter we have dealt with the properties 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 proceeding 

 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 con- 

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

 single spinal metamere, represented diagrammatically in Fig. 35, as illus- 

 trating 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 ner- 

 vous system C consists, as we shall hereafter see, of gray matter Gr in the 

 interior and white matter W on the outside. From the anterior part of 

 gray matter is given off the anterior nerve root A and from the posterior 

 part the posterior nerve root P. The latter passes into a swelling or gan- 

 glion G, " the ganglion of the posterior root," or more shortly " the spinal 



