38o STRUCTURE OF VERTEBRATES. 



The fourth region— that of the cerebellum, or metence- 

 phalon, or hind-brain— often has lateral lobes, and usually 

 overlaps the next region. The floor of the region of which 

 the cerebellum forms the roof, is known as the Pons Varolii. 



From the fifth region— that of the medulla oblongata or 

 myelencephalon, or after-brain— most of the cranial nerves 

 are given off. Its roof, partly overlapped by the cerebellum, 

 degenerates ; its cavity— called the fourth ventricle— is con- 

 tinuous with the canal of the spinal cord. 



Summary. 



) Cerebral hemispheres, prosence- 

 phalon, or fore-brain. Note 

 commissures, olfactory lobes 

 ^. ^ , . , and nerves, and first and second 



First Embryonic ) ventricles. 



vesicle. 'I ^2) Optic thalami, thalamencephalon, 



or tween-brain. Note {a) optic, . 

 {b) pineal, (<r) pituitary out- 

 growths, and the third ventricle. 

 ,, ,. „ , . ((■{) Optic lobes, mesencephalon, or 

 Median Embryonic y-'^ mid-brain. Note crura cerebri. 

 Vesicle. ^ ^^^ jj^g aqueduct of Sylvius. 



{(4) Cerebellum, metencephalon, or 

 hind-brain. Note Pons Varolii. 

 (5 ) Medullaoblongata, myelencephalon, 

 or after-brain. Note rudiment- 

 ary roof, fourth ventricle, and 

 origin of most of the cranial 

 nerves.- 



The pineal upgrowth or epiphysis from the roof of the 

 thalamencephalon is usually interpreted in one of two ways, 

 (i) As it is associated with what seems to be an unpaired 

 eye in the strange New Zealand reptile Sphenodon and in 

 some lizards (e.g.. Iguana)., it is likely that it may be in all 

 cases a rudimentary organ associated with a lost eye. (2) 

 But some maintain that the epiphysis is to be regarded as 

 a remnant of the pore which remained when the folds of the 

 medullary groove closed in anteriorly to form the brain. 



The pituitary downgrowth or infundibulum from the floor 



