BRAIN 



423 



and forcing the ends toward one another; the region with the weakened 

 dorsal wall buckles downward and bulges toward either side. The most 

 prominent part of the embryonic hind-brain, as it buckles downward, be- 

 comes the pons in the adult. From the dorsal part of the front end of 

 the hind-brain, the cerebellum develops, overhanging the thin roof of the 

 posterior portion. Pons and cerebellum are thus both derived from the 

 anterior part of the rhombencephalon, which is set apart as the meten- 

 cephalon; the remainder of the hind-brain is included in the myelencephalon 

 (Fig. 432), which becomes the medulla oblongata and is continuous with 

 the spinal cord. 



Before considering the subdivisions of the hind-brain in further detail, 

 the relation of the principal parts of the adult brain to the primary 

 vesicles may be reviewed in the following table: 



Fore-brain. . 



Telencephalon. 



Hemisphere : 



Pallium. 



Rhinencephalon. 



Corpus callosum. 

 Optic part of the hypothalamus. 



Hypophysis (posterior lobe). 



Pineal body. 

 Thalamus. 



Mammillary part of the hypothal- 

 amus. 



Mid-brain. . . { Mesencephalon . . . { Corpora quadrigemina. 



[ Cerebral peduncles. 



Diencephalon , 



Hind-brain. 



[ Isthmus Isthmus. 



Metencephalon...( CerebeUlim - 

 . I Pons. 



Myelencephalon. . Medulla oblongata. 



Metencephalon. The pons, as seen from the under side of the brain 

 (Fig. 435, B), appears as a broad bundle of transverse fibers interrupted 

 for the passage of the motor and sensory roots of the trigeminal nerve. 

 The superficial fibers of the pons pass dorsally around the wall of the 

 brain-tube, forming a pair of arms, the brachia pontis, which enter the 

 cerebellum. In addition to these large bundles, the cerebellum receives 

 fibers through the brachia conjunctiva which extend into it from the 

 isthmus, and also from the restiform bodies (i.e., rope-like) which ascend 

 from the posterior part of the hind-brain (Fig. 435, A). Thus on either 



