THE NERVOUS SYSTEM 



479 



ganglion cells of the dorsal nerves lie either in the dorsal wall of the cord 

 or are embedded in the nerves. The motor ganglion cells, as in verte- 

 brates, lie within the ventro-lateral wall of the cord. (Fig. 429, .4) 



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 ganglia. 



Cyclostomes. Compared with 

 cephalochordates, the cyclostomes 

 show a marked advance in the 

 complexity of their nervous system. 

 Instead of only two brain divisions, 

 cyclostomes have five, telence- 

 phalon, diencephalon, mesencepha- 



1 , 1, „ 1 „ r, r^ A Fig. 30 S- — Dorsal and lateral views of 



Ion, metencephalon, and ^^^ ^,^^^1^ Petromyzon planeri. Thetelae 



myelencephalon. Ontogenesis re- chorioideae removed, and the epiphysial 



1 1 „ i-Ur^t- fVi-ic-o f,-irc^ structures not shown in the side view, c, 



veals, however, that these five ^^^^^^^^^ ^^^^^. ^_ epiphysial structures; 



vesicles of cyclostomes and higher j^, saccular part of infundibulum-, /a, acoustic 

 , , , 1 1 r ,^ -v,^ ,^r■^r,^ lobc: Ik, left habenular ganglion; li, lobular 



vertebrates develop from the origi- p^^'^f -^f^^dibulum; iv, lobe of vagus; m, 



nal three which are correlated with mid-brain; of, olfactory bulb; pc. posterior 

 thp thrpp maior senses smell sieht commissure; rh, right habenular ganglion; 



tne tnree major senses, bmeii, bigiiL, ^^ ^^^^ ^^.^^^ ^^^^^. ^^^^ thalamus; 1-12, 



and hearing. The primitive fore- cranial nerves. (From Kingsley's "Com- 

 brain, which in the opinion of ^Xm.) ^"^'"""^ °^ Vertebrates." after 



most morphologists corresponds to 



the prosencephalon of amphioxus, becomes the telencephalon and dien- 

 cephalon, the midbrain continuing as the mesencephalon, and the original 

 hindbrain divides into metencephalon and myelencephalon. Since the 

 cyclostome brain may be taken as the complete prototype of that of all 

 vertebrates, and since most of its features persist in higher forms, these 

 are worthy of mention in some detail. (Fig. 395) 



