206 ANIMAL BIOLOGY 



It is usually considered that the primitive segmental condition 

 of the Vertebrate body is well exhibited in the arrangement of the 

 cranial and spinal nerves, and that the origin of the cranial nerves 

 from the brain affords a partial index to the primary series of seg- 

 ments which apparently have been merged to form the Vertebrate 

 head. Conditions as they exist at the present time can perhaps be 

 most readily understood by imagining a simple, ancestral, seg- 

 mented worm-like form in which the dorsal neural tube gives off 

 a pair of nerves to each segment of the body. As the result of a 

 gradual shifting forward, union, and finally complete fusion of 

 certain segments near the anterior end, there is formed a head 

 region with its brain, battery of sense organs, and skull, more or 

 less distinct from a trunk region with its spinal cord, vertebral 

 column, paired limbs, etc. This cephalization naturally involves a 

 shifting and modification of the primitive condition of the paired 

 nerves; especially since the innervation of a group of cells in nor- 

 mal development is apparently rarely changed — a nerve following 

 the part which it originally supplied through many of the trans- 

 formations and even migrations of the latter. 



If this point of view is accepted, the cranial and spinal nerves 

 are, historically considered, similar structures. But the former, 

 synchronously with the changes in the head region, have departed 

 somewhat widely from their ancestral condition and have even been 

 augmented by nerves of diverse origin. The spinal nerves, on the 

 other hand, continue to issue from the cord at about equal intervals 

 and in segmental arrangement as indicated by muscle segments 

 and skeletal structures, although those of certain regions unite in 

 the body cavity to form groups, or plexuses, to afford an adequate 

 nerve supply to the appendages. (Fig. 142.) 



From the standpoint of function the nerves are of three classes : 

 sensory, motor, and mixed. Sensory nerves are the paths over 

 which excitations (nerve impulses) due to stimuli are conducted 

 to the cord and brain, while motor nerves are the paths for dis- 

 tributing impulses from the brain and cord to muscle cells, gland 

 cells, etc., and thus induce the response of the organism. But the 

 great majority are mixed nerves which afford paths for sensory as 

 well as for motor impulses and so perform both functions. 



It is important to note that a nerve is actually a bundle of nerve 

 fibers; the fibers themselves in turn being prolongations of nerve 

 cells, the cell bodies of which are usually in groups, or ganglia. 



