THE NERVOUS SYSTEM 



481 



In addition to the numerous nervous structures which emerge in 

 the cyclostome brain, the hypophysis acquires intimate relations with 

 the infundibulum, and shows the first stages in the formation of the 

 pituitary gland, which is represented by a cluster of vesicles derived from 

 the hypophysis. 



TELEN- 



DIEN- 



epiphysis' 

 parietal organ, 

 habenular ganglion 

 dorsal sac 

 paraphysis 

 Monroe's foramen 

 hemisphere- 



MESEN- METEN- 

 ---" V ^ V 



.POST. COMMISSURE 



/TECTUM OPTICUM 



MYELENCEPHALDN 



SPINAL 

 [GANGLION 



BASAL GANGLION 

 ANT COMMISSURE 

 CHORIOID PLEXUS 



^ Y" uS?^ I NFUND I BULUM 



(OPTIC \ \hypophysis 



CHIASMA \qp^,(. thalamus 



LATERAL VENTRICLE/ 



BASAL GANGLION 



HEMISPHEREv 



OLFACTORY 

 LOBE I 



OPTIC THALAMUS 

 EPIPHYSIS 

 VENT HI I I /HABENULAR GANG. 



ANTPALUI 

 COMMISSURE 



ANT COMMISSU 



Fig. 396. — Diagrams of the vertebrate brain, based upon the brain of a cyclostome. 

 A shows the brain in median longitudinal section, with nerves as if projected upon the 

 median plane. B is the brain viewed from above. (Redrawn from Plate, after 

 Biitschli.) 



The nourishment of the brain is effected chiefly through the three 

 chorioid plexuses in the roof of diencephalon, mesencephalon, and 

 myelencephalon. 



The spinal cord in cyclostomes has become a thick-walled tube in 

 which three layers are differentiated, an outer marginal layer of fibers, 

 a middle mantle layer of gray matter, and a central ependymal layer 

 which lines the central canal. The gray matter has only two lateral 

 wings or columns instead of the four characteristic of higher vertebrates. 

 A number of giant nerve fibers like those of amphioxus extend along 

 the spinal cord, but they do not decussate (i.e., cross to the opposite 



