THE NERVOUS SYSTEHi 



507 



which become a pathway for fibers connecting anterior and posterior 

 parts of the brain. From the hypothalamus, the infundibulum is formed 

 as a ventral funnel-like outgrowth; and immediately behind this is a 

 second depression, the saccus vasculosus, limited to fishes and supposed 

 to be an hydrostatic pressure organ. Below the infundibulum, the 

 hypophysis proliferates vesicular masses which are believed to be homolo- 

 gous with the intermediate lobe of the pituitary gland. 



In elasmobranchs geniculate nuclei connected with fibers of the optic 

 nerves develop in the lateral portion of the thalami; hence these are often 

 called optic thalami. 



, PYRAMIDAL TRACT 



CORTICO-PONTAL TRACT 



CENTRAL GYRUS 



'LEMNISCUS TRACT 

 THALAMUS 

 r' PARIETAL LOBE 



VISUAL TRACT 

 -AUDITORY TRACT 



CAUDATE V_ 



NUCLEUS 



TEMPORAL LOBE— V-- 



OCCIPITAL U)BE 



-CEREBELLUM 



^MIDDLE PEDUNCLE 

 MNFERIOR PEDUNCLE 



Fig. 424. — A diagram showing the relations of the "projection" fiber tracts of 

 the human brain. The cranial nerves are numbered with Roman numerals. (Redrawn 

 after W. H. Howell.) 



The diencephalon of Amphibia differs Httle from that of elasmobranchs, 

 except that some amphibia have a rudimentary parietal eye which pierces 

 the skin or lies just beneath it. A chorioid plexus projects into the third 

 ventricle. The pituitary gland has the three lobes found in man. 



In reptiles, the chorioid plexus of the diencephalon grows forward 

 into the lateral ventricles, also. The epiphysis is glandular. The thick- 

 ened thalami nearly meet in the median plane and divide the ventricle 

 into dorsal and ventral chambers. 



The chorioid plexus of the diencephalon persists in mammals. The 

 epiphysis becomes the pineal gland, of problematic function, and there 

 is no parietal organ. The habenular ganglia are rudimentary. The 

 striate body of the telencephalon rests upon the thalamus, and the two 



