150 



Basic Structure of Vertebrates 



PINEAL NERVE 



MIDBRAIN 

 i POSTERIOR COMMISSURE 



C REPTILE 



SUPERIOR MABENULAR/ 



COMMISSURE 



POSTERIORI 

 COMMISSURE 



D MAMMAL 



Fig. 142. Parietal and pineal organs of vertebrates as seen in sagittal section. 

 (A) Cyclostome. (B) Anuran. (C) Beptile. (D) Mammal. Either the parietal or 

 the pineal organ may be more or less eyelike — in amphibians it is the pineal organ, 

 in reptiles the parietal organ. In man and other mammals the pineal organ is a 

 gland of doubtful function. (Bedrawn from Oppel, after Studnicka. Courtesy, 

 Neal and Band: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



of human anatomy) is the foremost of the median cavities of the brain, 

 each hemisphere of the telencephalon having its own lateral ventricle. 

 Each of these lateral ventricles communicates with the median dia- 

 coele via an aperture, the foramen of Monro (Fig. 138A). The anterior 

 wall of the diacoele is a thin lamina terminalis which terminates the 

 median cavities of the brain, the foramens of Monro being at either 

 side of the lamina (Fig. 139). 



The mesencephalon is thickened on all sides. The dorsal thicken- 

 ing assumes the form of a pair of prominent lobes, the optic lobes 

 (corpora bigemina: Fig. 136), or, in mammals, two pairs of lobes 

 (corpora quadrigemina: Fig. 140, ol). These optic lobes contain pri- 

 mary centers of the optic nerves. It is to be emphasized that, whereas 

 the retina develops as an outgrowth from the diencephalon and the 

 optic nerves join the floor of the diencephalon, the ultimate connec- 

 tions of the optic fibers are mainly in the mesencephalon. The floor of 

 the mesencephalon contains a pair of approximately longitudinal nerv- 

 ous tracts, the crura cerebri (cerebral peduncles: Fig. 530), which are 

 more or less prominent on the ventral surface. The optic lobes may 



