190 



FOUNDATIONS OF BIOLOGY 



the brain and cord to the other, and pass out through 

 openings in the skull and between or through the vertebrae 



to constitute the peripheral 

 nervous system. (Fig. 106.) 

 It is usually considered 

 that the primitive segmen- 

 tal condition of the Verte- 

 brate 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 in- 

 dex to the primary series 

 of metameres which ap- 

 parently have been merged 

 to form the Vertebrate 

 head. Conditions as they 

 exist at the present time 

 can perhaps be most readi- 

 ly 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 

 and a consequent coales- 

 cence and fusion of certain 

 segments near the anterior 

 end, there is brought about 

 the delineation of a head 



FIG. 106. Ventral view of the nervous 

 system of the Frog. Br, second and third 

 spinal nerves (brachial plexus); Js, sciatic 

 nerve leading from the sciatic plexus; O, eye; 

 Ol, olfactory nerve; Op, optic nerve; Sg 1-10, 

 ten ganglia of autonomic system; Spn 1, 

 first spinal nerve; Sp 4, fourth spinal nerve; 

 Vg, Gasserian ganglion; Xg, ganglion of 10th 

 cranial nerve (vagus). (After Ecker.) 



