314 NEUROLOGY, OR ANATOMY OF NERVOUS SYSTEM 



The corpora quadrigemina are seen on the dorsal surface of 

 the mid-brain, after the greater portion of the cerebral hemi- 

 spheres and other overlying structures have been removed. 

 They are four rounded eminences, placed in pairs, demarcated 

 by a crucial depression. The cephalic pair, the superior quad- 

 rigeminal bodies (pregemina or superior colliculi), are the 

 larger, and the pineal body lies between them. They are oval, 

 and of a yellowish-gray color. The superior quadrigemina 

 bodies, pass lateroventrad to end in the prebrachium, the latter 

 being continuous cephaloventrad between the overhanging 

 pulvinar, and the internal geniculate body. The inferior corpora 

 quadrigeminal bodies (postgemina or inferior colliculi) are lighter 

 in color than the former and passes lateroventrad to end in the 

 postbrachium, the latter continuing upward and forward to 

 pass beneath the internal geniculate body. 



The internal geniculate body, or postgeniculum, is a small 

 oval eminence of the lateral surface of the mid-brain in which 

 the internal fibers of the optic tract appear to terminate. 

 The inferior brachium or postbrachium runs into this body. 

 It is supposed to be a way station for auditory impulses in 

 their course to the cerebrum, also being the origin and terminus 

 of the arched commissure of Gudden. 



The aqueduct and central aqueduct gray are best seen on 

 a (microscopic) cross-section of the mid-brain through the 

 level of the superior quadrigeminal bodies. The aqueduct is a 

 narrow canal connecting the third with the fourth ventricle. 

 It demarcates the lamina quadrigemina dorsad from the 

 tegmental zone, and is surrounded by the central aqueduct 

 gray, which contains the nuclei of origin of the oculomotor, 

 trochlear, and the mesencephalic root of the trigeminal nerves. 



SUMMARY OF THE GRAY MASSES IN THE MID-BRAIN 



Central aqueduct gray : (a) Oculomotor nerve nucleus. 

 (6) Trochlear nerve nucleus. 

 Nucleus radicis descendentis nervi trigemini. 

 Nucleus of medial longitudinal bundle and postcommissure. 

 Formatio reticularis. 

 Substantio nigra (intercalatum). 

 Red nucleus (rubrum). 



