422 



HISTOLOGY 



corpora quadrigemina (Fig. 435, A). These are arranged in pairs, the 

 anterior pair being known as the superior colliculi, and the posterior as 

 the inferior colliculi; the former have important relations with the optic 

 tracts, and the latter with the auditory tracts. On the under side of the 

 mid-brain there are two great bundles of fibers, the cerebral peduncles 

 (pedunculi cerebri) , which diverge as they pass forward from the hind-brain, 

 and swing upward on the sides of the mid-brain to connect with the hemi- 

 spheres (Fig. 435). Between the cerebral peduncles on the under side of 

 the mid-brain, the oculomotor nerves emerge. They are derived from 

 groups of motor cells situated just beneath the floor of the cavity of the 

 mid-brain. This cavity remains a slender tube and is known as the 

 cerebral aqueduct (aquaductus cerebri). 



i : iS^3^ oc. 



FIG. 433. A, DORSAL AND B, VENTRAL VIEW OF THE POSTERIOR PART OF THE ADULT BRAIN. THK 

 CEREBELLUM AND ROOF OF THE FOURTH VENTRICLE HAVE BEEN REMOVED FROM A. 



b. c., Brachium conjunctivum; b. p., brachium pontis; c. m., corpus mamillare; c. p., cerebral peduncle; 

 c. q. a., and c. q. p., anterior and posterior corpora quadrigemina; inf., infundibulum; med., medulla; 

 ol., olive; p., pons; p. b., pineal body; pyr., pyramid; r. b., restiform body; ven., floor of fourth ventri- 

 cle. The nerves are oc., oculomotor; tr., troclear; tri., trigeminal; abd., abducens; int., intermedius, 

 fa., its facial portion; ac., acoustic; glo., glossopharyngeal ; va., vagus, ace., its accessory portion; 

 hy., hypoglossal. 



Between the mid-brain and the hind-brain there is a marked constric- 

 tion, known as the isthmus (Fig. 432, B). From the dorsal surface of the 

 isthmus the trochlear nerves make their exit (Fig. 435, A); they are 

 processes of nerve cells situated beneath the floor of the cavity, but they 

 pass to the dorsal surface and cross to the opposite side before emerging. 



Rhombencephalon. The rhombencephalon (or hind-brain) receives its 

 name from the diamond shape which it presents when seen from above. 

 This form is established in young embryos and persists in the adult (Fig. 

 435, A). The roof of the rhombic cavity becomes a thin membrane and 

 is readily torn away, but the sides and especially the floor are greatly 

 thickened. The form of the hind-brain may be imitated, as described 

 by His, by cutting a short slit in the upper side of a piece of rubber tubing 



