THE FORE-BRAIN OR PROSENCEPHALON 



807 



cells of various sizes, interlaced, by a net-work of fine fibers. Besides these scattered 

 cells it contains three groups which constitute the nuclei of the oculomotor and 

 trochlear nerves, and the nucleus of the mesencephalic root of the trigeminal nerve. 

 The nucleus of the trigeminal nerve extends along the entire length of the aqueduct, 

 and occupies the lateral part of the gray stratum, while the nuclei of the oculo- 

 motor and trochlear nerves are situated in its ventral part. The nucleus of the 

 oculomotor nerve is about 10 cm. long, and lies under the superior colliculus, beyond 

 which, however, it extends for a short distance into the gray substance of the third 

 ventricle. The nucleus of the trochlear nerve is small and nearly circular, and is on 

 a level with a plane carried transversely through the upper part of the inferior 

 colliculus. 



THE FORE-BRAIN OR PROSENCEPHALON. 



The fore-brain or prosencephalon consists of: (1) the diencephalon, corresponding 

 in a large measure to the third ventricle and the structures which bound it; and 

 (2) the telencephalon, comprising the largest part of the brain, viz., the cerebral 

 hemispheres; these hemispheres are intimately connected with each other across 

 the middle line, and each contains a large cavity, named the lateral ventricle. 

 The lateral ventricles communicate through the interventricular foramen with the 

 third ventricle, but are separated from each other by a medial septum, the septum 

 pellucidum; this contains a slit-like cavity, which does not communicate with the 

 ventricles. 



FORAMEN OF MONRO 





OCULOMOTOR X 

 NERVE 



CORPUS ALBICANS 

 TUBER CINEREUM 



OSTRUM / 



COPULA 



ANTERIOR 

 COMMISSURE 

 LAMINA TERMINALIS 



OPTIC CHIASM 



NERVE 

 PITUITARY BO6Y 



QUADRIGEMINAl 

 LAMINA 





AQUEDUCT 



N^SUP. MEDULLARt 



VELUM 

 \FOUHTH 



VENTRICLE 



FIG. 715. Mesal aspect of a brain sectioned in the median sagittal plane. 



The Diencephalon. The diencephalon is connected above and in front with 

 the cerebral hemispheres; behind with the mid-brain. Its upper surface is con- 

 cealed by the corpus callosum, and is covered by a fold of pia mater, named the 

 tela chorioidea of the third ventricle; inferiorly it reaches to the base of the brain. 



The diencephalon comprises: (1) the thalamencephalon; (2) the pars mamillaris 



