292 



STRUCTURE OF THE VERTEBRATES 



portion of the ventricle is a small tube, the iter or aqueduct oj 

 Sylvius. 



jMetencephalon. The meteneephalon is the anterior portion 

 of the hindbrain. From its dorsal region develops the cerebellum, 

 the major seat of the nerves influencing equilibrium and coordi- 

 nation in the vertebrate. 



The cerebellum is the first brain region to develop a cortical 

 zone. In the primitive animals it is relatively small, with few 

 fibers on the ventral side. In recent amphibia the cerebellum 

 has undergone an interesting degenerative specialization, the 



Corpora quadrigemina 



Optic lobes 



Cerebellum 



Medulla 



B. Lizard 



C. Mammal 



A. Dogfish 



Fig. 165. Midbrains and Hindbrains of Dogfish, Reptile and Mammal. The 

 midbrain of the mammal is divided transversely to form the corpora 



quadrigemina. 



region being very small, with a consequent loss of well coordi- 

 nated movements. The higher reptiles and the birds have the 

 central region of the cerebellum well developed, with lateral 

 outgrowths (the flocculi) which increase the functional size of 

 the structure. 



The mammals add two lobes to the cerebellum. The original 

 lobe becomes transversely creased and is known as the vermis, 

 with the lateral lobes on either side. The flocculi remain, but are 

 relatively less important than in the birds. The ventral part of 

 the cerebellum is a great mass of transverse myelinated fibers, 

 the pons Varolii, or more usually simply pons. 



