72 ANATOMY OF THE RABBIT 



sisting of nerves which can be seen ramifying through all parts of 

 the body, than of the central portion comprising the brain and 

 spinal cord. In examining the external form as a preliminary step 

 to the study of the functional arrangements, it is advisable to bear 

 in mind that the nervous system is a great correlating mechanism, 

 consisting of centres where exceedingly complex inter-connections 

 are made between the nerve elements, and of conducting paths to 

 and from these centres connecting them with outlying parts of the 

 body. 



In accordance with its prime importance and at the same time 

 the non-resistant character of the tissue of which it is composed, 

 the central nervous system is protected within the canal of the 

 vertebral column and cavity of the brain-case. It is furthermore 

 surrounded by connective tissue membranes, meninges. In higher 

 vertebrates, three of these are differentiated, the dura mater, 

 which forms a tough external investment; the arachnoidea, which 

 is a very delicate, somewhat spongy web lying internal to this; 

 and the pia mater, a thin membrane lying next the nervous matter 

 and richly supplied with blood-vessels from which branches pene- 

 trate the latter to provide for its nourishment and respiration. 

 Still further protection is afforded by the cerebrospinal fluid, 

 which fills the spaces between these membranes as well as the 

 cavities within the brain. The nerves, on the other hand, are 

 distributed freely throughout the body, and though not so ade- 

 quately protected are more capable of withstanding or repairing 

 mechanical injury. Also they are commonly found surrounded by 

 connective tissue where mechanical injury is relatively little likely 

 to occur. Each nerve has a tough sheath, the perineurium, com- 

 posed of condensed connective tissue, and similar tissue extends 

 between the fibres, binding them together in small bundles. Each 

 has a relatively poor though indispensable blood supply received 

 through small arteries which anastomose to form a continuous 

 channel along the nerve. 



The central nervous organs contain numerous blood vessels, 

 the capillaries forming a continuous network the density of which 

 differs considerably in different parts, but they have no lymphatics. 

 The tissue spaces are continuous with narrow perivascular spaces 

 which are nowhere lined with endothelium as are lymphatics. 



