584 Dynamic Theory. 



lobes are in contact with each other, and are oblong, smooth, white vesi- 

 cles of nerve matter. These characters, and the rudimentary state of 

 the cerebellum, are representatives of the permanent condition of the 

 brain of the adult batrachians. At the end of the sixteenth day, the 

 brain of the chick has acquired the appearance represented in fig. 300, 

 and resembles the brain of the Lizzard. (Fig. 292. ) The cerebellum 

 has grown over the fourth ventricle ; its surface is smooth, as in the 

 Turtle and Crocodile ; the optic lobes have become spheroidal in shape, 

 and the hemispheres cover up the optic thalami. About the time of 

 hatching, the brain has the appearance represented by fig. 301. The 

 cerebellum has largely increased, grown forward between the corpora 

 bigemina, pushing them apart, and its surface covered with gray mat- 

 ter is corrugated by transverse folds. In the mature bird, there is a 

 rudimentary fissure of Sylvius, a mere depression where that fissure ex- 

 ists in the mammals. The hemispheres are connected together by the 

 anterior or round commissure. The corpora striata are relatively very 

 large in birds, and form, in fact, the greater part of the cerebrum, but 

 they do not possess the alternate striations of white and gray, shown in 

 the higher mammals. In the latter respect they agree with adult rep- 

 tiles and the embryo of the mammalia. The lateral ventricles have no 

 descending cornu. They are separated from each other by a thin par- 

 tition of medullary matter. Beneath the posterior part of this wall 

 there is an orifice by which the two ventricles communicate with each 

 other and also with the third ventricle. Just above the orifice is a 

 small projection of medullary matter, which is an undeveloped begin- 

 ning of the fornix. The optic thalami are small and not connected by 

 the commissure, as in the Lizzard. The pineal gland is attached be- 

 hind to the valve of Vieussens, and its peduncles connect it with the 

 optic thalami. The corpora bigemina, or optic lobes, have ventricles, 

 which open into the aqueduct of Sylvius. The third ventricle opens be- 

 low into the infundibulum, at the bottom of which is a large pituitary 

 gland. The cerebellum has but one lobe, the median, which answers 

 to the vermiform process of the higher mammals, while the lateral lobes 

 are wanting, or nearly so, and there is no pons varolii. The cerebrum 

 and cerebellum of Birds are superior in size to those of the Reptiles, 

 while the cerebellum is superior in structure by reason of its folds. 

 There is great relative difference in the sizes of the brains of birds, but 

 it chiefly relates to the optic lobes. The cerebral hemispheres remain 

 small and unconvoluted in all. ( Cuvier. ) 



Mammals. Owen proposes a classification of mammals, based upon 

 their brain development. In this classification the monotremes and 

 marsupials are placed as a sub-class at the bottom of the series, and are 

 named Lyencephala (loose-brained). The fig. 304 of the brain of the 



