RESPONSIVENESS IN VERTEBRATES 



and medulla oblongata, respectively, together with the highly vascularized pia 

 mater, the innermost of the membranes surrounding the brain. These rich 

 vascular beds are carried into the cavities of the third and fourth ventricles 

 by the downgrowth of the thin-walled regions. An extension of the anterior 

 plexus passes into the ventricle of each cerebral hemisphere. The cavity of 

 the central nervous system is filled with the cerebrospinal fluid, which is chiefly 

 filtered from the blood stream in the plexuses. This fluid also surrounds the 

 brain and spinal cord and has a cushioning, protective function. A slow cir- 

 culation is maintained, since the cerebrospinal fluid returns to the blood 

 stream by way of the venous channels in the outermost of the brain mem- 

 branes, the dura mater. 



The Peripheral Nervous System. The peripheral nervous system is com- 

 posed of the cranial, spinal, and autonomic nerves (Fig. 4.2). These are 

 bundles of nerve fibers (p. 68) which originate from nerve cell bodies located 

 in the gray matter of the central nervous system or in ganglia which are 

 groups of functionally related nerve cell bodies outside the central nervous 

 system. In the brain there are groups of nerve cell bodies similar to the 

 ganglia but called centers. In the lower vertebrates, such as the frog, there 

 are ten pairs of cranial nerves which are continuous with the brain. All pass 

 to the head and neck with the single exception of the tenth pair, the vagus 



Vitreous humor 

 Blind spot 



contain 



Aqueous 



humoi" 



Suspensory 

 ligament 



Sclera 



Fig. 4.3. The human eye in section (cf. Fig. 4.13). 



89 



