PITHECUS RHESUS, MACACUS RHESUS ' 363 



Iinnu'diately above the aqueduct appears the quadii<i;emiiial phite ot the mid- 

 brain, characteristically arranged in four symmetrical elevations, two of 

 which are caudal in position, the inferior colliculi, while the two cephalic 



FIG. 170. DORSAL SURFACE OF BRAIN STEM, MACACUS RHESUS. 



[Actual Length, 44 mm.] 



Key to Diagram, clav., Clava; d. m. fis., Dorsomedian Fissure; d. med. septum, Dorsomedian Septum; 



INF. COLL., Inferior Colliculus; sup. cer. peduncle and s. c. ped., Superior Cerebellar Peduncle; sup. coll., 



Superior Colliculus; tub. tricem., Tuberculum Trigemini. 



elevations form the superior colliculi. These latter are about twice the 

 size of the inferior colliculi, and slightly more elevated as tectal structures. 

 The four elevations are separated longitudinally by the median intercol- 

 licular sulcus, and transversely by the transverse intercollicular sulcus. On 

 the lateral surface of the midbrain, extending from the inferior colliculus, 

 is the brachium conjunctivum posticum which terminates in the mesial 

 geniculate body. A similar brachium extends forward from the superior 

 colliculus to terminate in the lateral geniculate bodv. 



