712 



Comparative Morphology of Chordates 



pair are mainly auditory. (In some reptiles and in birds there is a pair 

 of very small lobes just behind the corpora bigemina.) 



It is to be noted that, whereas the external connection of the optic 

 nerves with the brain is at the base of the diencephalon, the internal 

 connections of the fibers of the optic nerves are largely, or in fishes 

 perhaps entirely, in the roof of the mesencephalon. The position of the 

 external connection of the nerves with the brain is an incident of the 

 fact that the retina develops as an outgrowth from the base of the 

 first (anterior) primary brain vesicle (see p. 200). 



In the metencephalon the cerebellum exhibits conspicuous 

 mammalian peculiarities (Figs. 521, 524). Relatively large, it is divided 

 into a median lobe and a pair of lateral lobes or cerebellar hemi- 

 spheres. Lateral and ventral to each hemisphere is a very small lobe, 

 the flocculus. The flocculi correspond to the similarly named bodies 

 found in birds and some reptiles. The surfaces of the cerebellar lobes 



HYPOPHYSIS 



ANTERIOR 



PERFORATED 



SUBSTANCE 



OPTIC CHIASMA 



TUBER CINEREUM 



MAMMILLARY BODIES 



ATERAL GENICULATE 

 BODY 



CEREBELLUM 



DECUSSATION OF PYRAMIDS 



SPINAL CORD 



Fig. 530. Human brain-stem in ventral aspect, showing nerve connections. 

 (After Morris and Thomson. Courtesy, Neal and Rand: "Chordate Anatomy," 

 Philadelphia, The Blakiston Company.) 



