NERVOUS SYSTEM 153 



bellum and medulla oblongata, however, remain somewhat 

 primitive in type. 



Characteristic of the reptiles are the two curves of the brain, 

 one occurring where the second vesicle separates from the 

 third, the other in the neighbourhood of the medulla oblongata ; 

 the separate divisions of the brain also increase in volume and 

 in complexity of structure. The cerebrum with its two hemi- 

 spheres covers the second vesicle and the lateral ventricles are 

 well developed. The third vesicle (mesencephalon) is divided 

 into two hemispheres by a deep fissure. In lizards and snakes 

 the cerebellum is little developed, being more clearly differen- 

 tiated in the tortoise and crocodile ; in the latter a median 

 protuberance is distinguishable from the two lateral ones. 



In birds the cerebrum is more complex ; it consists chiefly of 

 ganglion cells of the corpora striata, and with its hemispheres 

 covers the small second vesicle. Here, too, the mesencephalon, 

 or third vesicle, which is still of considerable size in the embryo, 

 is divided into two halves as in the reptiles ; the cerebellum, 

 however, is, in its middle part, still more highly developed than 

 in the crocodile and almost completely covers the medulla 

 oblongata. 



A glance at the embryonic brain of the mammals, with its 

 manifold points of resemblance to lower forms, will show that 

 we are not justified in assuming that an essential difference 

 exists between the brain of the mammals and that of the lower 

 Vertebrates. At a later stage of development extensive 

 modifications take place, it is true, especially as regards the 

 cerebrum, where the two hemispheres, separated from each 

 other by a deep fissure, overlap the two olfactory lobes and are 

 connected by a broad band of nerve tissue, the corpus cal- 

 losum. 



Proceeding from the Monotremata up to the higher forms 

 of life we find the cerebrum steadily encroaching on the 

 mesencephalon, till finally, in the apes' and man, it covers the 

 cerebellum. The second vesicle becomes the optic thalamus, 

 the third the corpora quadrigemina, and the fourth, or posterior, 

 develops into the relatively important cerebellum, the lateral 

 lobes of which are developed, especially in the apes and man, 

 at the expense of the middle lobe. 



