THE NERVOUS SYSTEM 



349 



The deuterencephalon is possibly homologous with the mid- and 

 hind-brain of vertebrates. It differs from the spinal cord in having in its 

 dorsal wall large neurosensory cells known as cells of Joseph. Two 

 paired dorsal sensory nerves — numbered II and III — connect with the 

 deuterencephalon. The first pair of motor nerves connect with its 

 ventral wall. (Fig. 331) 



In Amphioxus dorsal and ventral nerves alternate with one another 

 throughout the length of the body. 

 Except the two anterior pairs, 

 which are wholly sensory, the dorsal 

 nerves of Amphioxus are mixed in 

 function. They extend between 

 the myotomes to the skin where 

 they divide into dorsal and ventral 

 rami. Ventral nerves, on the other 

 hand, pass from the cord directly 

 to the myotomes opposite. Con- 

 sequently, in cephalochordates, dor- 

 sal and ventral nerves do not 

 unite. The ganglion cells of the 

 dorsal nerves lie either in the dorsal 

 waU of the cord or are embedded 

 in the nerves. The motor ganglion 

 cells, as in vertebrates, lie within 

 the ventro-lateral wall of the cord. 

 (Fig. 326, A) 



On the basis of their peripheral 

 distribution, four kinds of nerve 

 fibers are distinguished, somatic 

 motor and somatic sensory, visceral 

 motor and visceral sensory. Each 

 somatic motor nerve innervates 

 three successive myotomes, but 

 most of its fibers pass to the middle 

 one. Giant ganglion cells occur in 

 the mid-dorsal line of the cord at 

 the anterior and posterior parts 

 of the body, but are wanting in the 

 intermediate region. Since these 

 connect with the sensory nerves, they are probably elements in a reflex 

 system. Sympathetic nerve fibers connect with the blood-vessels and the 

 viscera, but there is no chain of sympathetic gangUa. 



Cyclostomes. Compared with cephalochordates, the cyclostomes 

 show a marked advance in the complexity of their nervous system 



Fig. 309. — Dorsal and lateral views of 

 the brain of Petromyzon planeri. The telae 

 chorioideae removed, and the epiphysial 

 structures not shown in the side view, c, 

 olfactory lobes; e, epiphysial structures; 

 is, saccular part of infundibulum; la, 

 acoustic lobe; Ih, left habenular ganglion; 

 //, lobular part of infundibulum; Iv, lobe of 

 vagus; m, mid-brain; of, olfactory bulb; pc, 

 posterior commissure; rh, right habenular 

 ganglion; s, first spinal nerve; th, thalamus; 

 1-12, cranial nerves. (From Kingsley's 

 "Comparative Anatomy of Vertebrates," 

 after Ahlbom.) 



