02 THE NERVOUS SYSTEM AND ITS CONSERVATION 



i> an appreciable space containing a clear, limpid liquid, 

 the cerebrospinal fluid. Strictly speaking, a third mem- 

 brane, the nniclinoiil, intervenes between the dura and the 

 fluid, which is therefore, said to occupy the subarachnoid 

 space. The membrane- with the fluid retained by them 

 an- collectively spoken of as the nuniinji >-. 



The regular fashion in which the spinal nerves part 

 into dorsal and ventral roots might well suggest a differ- 

 ence in fund ion corresponding to the anatomic separation. 

 It has, in fact, been known for about a hundred years that 

 the dorsal roots are almost wholly composed of afferent 

 fibers and the ventral roots even more dominantly of 

 those which are efferent. The Matement of this contrast 

 in function between the roots of the two sets is known as 

 Bell's law. The cranial nerves do not part in this orderly 

 way into dorsal and ventral divisions where they join the 

 substance of the brain; moreover, they are extremely 

 unequal in size. Some are found to be composed of 

 afferent fibers, some of efferent, and some of both together. 

 It will be evident that each spinal nerve, if tested beyond 

 the point of blending of its two roots, may be expected to 

 show both characters. Most nerves are mixed in the 

 sense that they contain fibers of both kinds. In the sum 

 of all the nerves it is said that the afferent fibers are dis- 

 tinctly more numerous than the efferent. In the spinal 

 nerves the difference is moderate; in the cranial, the 

 preponderance of afferent libers is overwhelming. 



A <TO .eel ion of the spinal cord brings to view several 



features which may be described later, but at the moment 

 only one of these will be mentioned. This is the minute 

 central cmml, a reminder thai the central nervous system 

 i< to IK- regarded as tubular in its fundamental organi/a- 

 tion, though the walls of the primitive lube have en- 

 croaehed upon its cavity until the remaining space is 

 very small compared with the ma es of white and gray 

 matter surrounding it. The slender central canal of the 

 cord lead- forward to a much more con-piciioii- -cries of 

 chamber- in the brain. \Ye can now outline the anatomy 



