384 ELEMENTARY ANATOMY. [LESS. 



and in all below. In other words, a pons Varolii is a struc- 

 ture peculiar to man's own class. 



1 6. The MEDULLA OBLONGATA of man is normal, but less 

 complex than in much lower forms. 



Even in some members of man's own order (e.g. the 

 American Apes) certain additional structures (called corpora 

 trapezoidea) separate the upper ends of the pyramids from 

 the pons Varolii. 



The side walls of the fourth ventricle may be remarkably 

 contorted (as in Rays), forming what is called corpora resti- 

 formia (Fig. 336, 6). 



The little extension backward, or the rudimentary condition 

 of the cerebellum, sometimes causes the fourth ventricle to 

 come plainly into view, as we see in the Frog and the Lam- 

 prey. The fourth ventricle may be relatively enormous, as in 

 Hexanckus. 



17. Thus it seems that man exaggerates those characters 

 which distinguish Mammals from lower Vertebrates ; i.e. in 

 him the cerebrum and cerebellum are advanced to a maxi- 

 mum of relative size and complexity, while the parts which 

 represent those primitive segments, the fore-brain and mid- 

 brain, show an extreme simplicity and relative inferiority of 

 size. Man's brain is an example of one of three types of 

 structure found in the Vertebrate sub-kingdom. 



FIG. 338. LEFT SIDE VIEW OF BRAIN OF PIGEON (Colutnba li-via.~) 



i, olfactory lobe ; 2, cerebral hemispheres : 3, pineal gland ; 4, one of the optic 

 lobes (here lateral and depressed) ; 5, cerebellum ; 6, pituitary body ; 8, optic 

 nerve. 



The two other types are (i) the Sauropsidan brain and (2) 

 the Piscine brain. In the first we meet with hollow hemi- 

 spheres, large optic lobes (superior, or lateral and depressed) 

 no lobi inferiores, no enlarged roof to the third ventricle, and 

 no fold of brain-substance extending into a cavity beneath the 

 so-called optic lobes, while often a well-developed cerebellum 

 exhibits the arbor vitas. In the second type we meet with 



