2 20 



ELEMENTS OF MAMMALIAN ANATOMY 



The corpora quadrigcmina (Fig. 107) forming the dorsal part 

 of the mesencephalon lie craniad of the medulla, and consist 

 of an anterior pair and a posterior pair. The former are known 

 as the superior coUicidi and the latter as inferior colIicuH. The 

 anterior pair lie nearer to the midline than the posterior pair, 

 which are slightly separated by a depression occupied by the 

 middle portion of the central lobe of the cerebellum. The 

 posterior pair are united by a white commissure. The pos- 

 terior commissure of the brain unites the cranial portions of 



Fig. 108. — Cross-section of the Brain in the Plane x in Fig. 105. 

 The plane is just caudad of the optic chiasm. 

 I and 2, First and second or lateral ventricles: cr, corpus callosum; cm, columns 

 of fornix; ex, choroid plexus of lateral ventricle; ex', choroid plexus of third ven- 

 tricle; cxt, gray cortex; ea, ependyma lining the ventricles;/, median longitudi- 

 nal fissure; h, taenia thalami; me, middle commissure, or massa intermedia; 

 n, fibers of the optic tract as they enter the lateral geniculate body s; o, fornix; 

 r, caudal portion of nucleus lenticularis; op, thalamus; oc, optic tract as it leaves 

 the chiasm; si, septum pellucidum; vn, part of third ventricle dorsad to the 

 commissure; v, third ventricle; /, fibers of optic tract. 



the anterior pair (Fig. 106). Its cut end may be seen ventrad 

 to the base of the pineal gland. 



Laterally each of the corpora quadrigcmina is prolonged 

 into two white bands, the anterior and posterior hrachia. The 

 latter are about a half centimeter long, and pass forward beneath 

 a pisiform ganglion, the corpus geniculattim internum or mediate. 

 The anterior brachia pass from the cranial end of the posterior 

 pair laterad beneath the caudal projections of the thalami, 

 where they join the optic tracts. 



