THE TELENCEPHALON 



847 



the thalami. This greater width is occasioned by a groove in the ventromesial 

 surface of each thalamus, known as the hypothalamic sulcus (sulcus of Monro). 

 It is along the line of this sulcus that the third ventricle is continuous with the 

 aqueduct of the cerebrum, and thus with the fourth ventricle below, and, likewise, 

 with the two lateral ventricles of the cerebral hemispheres at its anterior end. 

 The latter junction occurs through a small oblique aperture, the interventricular 

 foramen (foramen of Monro), one into each lateral ventricle. The dorsal or 

 upper portion of the third ventricle extends posteriorly beneath its chorioid tela 



Fig. 668. — Ventral Aspect of Brain-stem Including Mammillary and Optic Portions 



OF the Hypothalamus. 



Insula 



^^^' Olfactory tract 



Hypophysis 

 1 ' 



Anterior perforated 

 substance — 



Mammillary bodies_ 



Cerebral peduncle 



Semilunar (Gasser 

 ian) ganglion 



Oblique fasciculus 

 of pons 



Optic nerve 

 Optic tract 



- Tuber cinereum 



^ Oculomotor nerve 



Lateral geniculate 

 body 



Hypoglossal nerve *' 



Pyramid' 



Decussation of pyramids ' 



Accessory nerve 

 spinal accessory) 



,-- Cervical II 



(velum interpositum) to form a small posterior recess about the epiphysis. This is 

 known as the supra-pineal recess. The anterior and ventral extremity of the 

 third ventricle involves the pars optica hypothalami, which belongs to the 

 telencephalon. 



B. THE TELENCEPHALON.— External features.— The optic portion of the 

 hjTJOthalamus consists of that small central area of the basal surface of the telen- 

 cephalon which includes and surrounds the optic chiasma, and comprises the 

 structures of the floor of the anterior and ventral portion of the third ventricle. 

 The area extends anteriorly from the mammillary bodies in the interpeduncular 

 fossa, and includes the tuber cinereum and hypophysis behind the optic chiasma, 

 and some of the anterior perforated substance in front of it. 



The most anterior portion of the third ventricle is in the form of a ventral ex- 

 tension. The wall of this portion is almost wholly non-nervous and quite thin, 

 and thus the cavity of the ventricle is but thinly separated from the exterior of 



