586 HANDBOOK OF PHYSIOLOGY. 



This division of the brain into the four parts is justified by a consid- 

 eration of its development. As will be seen later on, the brain consists 

 originally of three cerebral vesicles, the dilated extremity of the neural 



Fig. 357. View of the Corpus Callosum from above. ^. The upper surface of the corpus 

 callosum has been fully exposed by separating the cerebral hemispheres and throwing them to 

 the side ; the gyrus f ornicatus has been detached, and the transverse fibres of the corpus callo- 

 sum traced for some distance into the cerebral medullary substance. 1, the upper surface of the 

 corpus callosum; 2, median furrow or raphe; 3, longitudinal striae bounding the furrow; 4, 

 swelling formed by the transverse bands as they pass into the cerebrum ; 5, anterior extremity 

 or knee of the corpus callosum ; 6, posterior extremity ; 7, anterior, and 8, posterior part of the 

 mass of fibres proceeding from the corpus callosum ; 9, margin of the swelling ; 10, anterior part 

 of the convolution of the corpus callosum; 11, hem or band of union of this convolution; 12, in- 

 ternal convolutions of the parietal lobe ; 13, upper surface of the cerebellum. (Sappey after 

 Foville.) 



canal, and these consist of fore-, mid-, and hind-brain. From the fore- 

 brain there is first of all budded off on either side a new vesicle, the 

 optic vesicle from which is developed the optic nerve and retina, and 

 afterward a large vesicle, the cerebral vesicle, which grows rapidly, 

 becomes divided by a central partition into two, each of which incloses 

 the lateral ventricle. The cerebral vesicles grow so quickly as to cover 

 both the fore- and the mid-brain. The parts of which the fore-, mid-, 

 and hind-brains are made up are developed from the corresponding cere- 

 bral vesicles. 



It will be as well here to indicate briefly the structure of the brain. 

 It consists of white and gray matter differently arranged in different 

 districts. 



