THE STRUCTURAL AND FUNCTIONAL SYSTEMS 



65 



general, since its organization is intermediate between the 

 simpler conditions found in the fishes and the more specialized 

 relationships existing in reptiles, birds, and mammals. As seen 

 by dissection from its dorsal aspect, the brain of the frog 

 (Fig. 38 A) is composed of five main parts. The anterior portion of 

 the brain consists of the paired cerebral hemispheres, or cerebrum, 

 from which the olfactory lobes are incompletely separated. Pos- 

 terior to the cerebrum is a narrowed area, the diencephalon, 

 from the dorsal surface of which there arises a rudimentary struc- 

 ture, the pineal body , which is of interest because it is developed 



Fig. 38. — Brain of the frog. 



A, dorsal. B, in horizontal section to show internal cavities or ventricles. C, ventral. 

 cbl, cerebellum; cbm, cerebrum; ch.p., posterior choroid plexus; di, diencephalon; inf, 

 infundibulum; l.v., lateral ventricle; jnd, medulla oblongata; o.c, optic chiasma; oil., 

 olfactory lobe; ol.n., olfactory nerve; op.L, optic lobe; op.v., optic ventricle; pin, pineal 

 gland; pit, pituitary gland; s.c, spinal cord; s.n.l, 1st apinal nerve; sp.c, spinal cord; 

 D.3, 3d ventricle; vA, 4th ventricle; 3, 4, 5, 7, 8, 0, 10, cranial nerves. 



as a simple eye lying on the dorsal midhne in some of the rep- 

 tiles. The optic lobes are a pair of conspicuous rounded bodies 

 behind the diencephalon. Posterior to these is a narrow trans- 

 verse ridge, the cerebellum, followed by the medulla oblongata, 

 which is continuous with the spinal cord. 



When the brain is viewed from the ventral side (Fig. 38 C), one 

 finds, in addition to the parts above mentioned, the optic chiasma, 

 or region of the crossing of the optic nerves, in the anterior 

 ventral wall of the diencephalon; and the infundibulum, 

 extending downward from the posterior ventral wall of the 

 diencephalon to meet the hypophysis, which is not a part of 



