THE TELENCEPHALON 667 



which appear to ho withhi the ventricle, although they are excluded from the cavit}' 

 by the epithelium. When the tela is removed, the delicate ependyma of the 

 roof is torn away with it, heaving the line of attachment to the stria meduUaris to 

 constitute the taenia thalami. The anterior wall is formed by the lamina terminalis 

 (s. cinerea), a thin layer of gray matter which extends upward from the optic 

 chiasm to the corpus callosum. A distinct rounded l)and of white matter extends 

 across its posterior face, l)ulging into the ventricle. This is the anterior comniissure 

 (Commissura nasalis) of the cerebrum; its fibers extend to the olfactory bulb and 

 to the pj^riform lobe. A similar but more slender posterior commissure (Commis- 

 sura caudalis) crosses the posterior wall above the entrance to the aciueduct; 

 the connections of its fibers are not yet clearly known. The interventricular 

 foramen (of Monroe) is situated on either side of the anterior part of the ventricle 

 and leads outward and slightly upward between the anterior pillar of the fornix and 

 the anterior tu])ercle of the thalamus. The cavity presents three recesses or 

 diverticula, of which two are ventral and the third is supero-posterior. The 

 optic recess ( Hecessus opticus) lies above the optic chiasm. Just behind it is the 

 infundibular recess (Recessus infundil)uli) which extends through the infundibu- 

 lum to the pituitary body. The pineal recess (Recessus pinealis) is in the stalk of 

 the pineal l)ody. 



THE TELENCEPHALON 

 The telencephalon or end-brain comprises two principal parts, the cerebral 

 hemispheres and the oj^tic part of the hypothalamus. The latter has been con- 

 sidered as a matter of convenience in the description of the diencephalon. 



The Cerebral Hemispheres 



The cerebral hemispheres (Hemisphsria) form the greater part of the fully 

 developed l)rain. \'iewed from above (Fig. 498) they form an ovoid mass, of 

 which the broader end is posterior, and the greatest transverse diameter is a little 

 behind the middle. The two hemispheres are separated by a deep median cleft, 

 the longitudinal fissure of the cerebrum, which is occupied by a sickle-shaped 

 fold of dura mater, the falx cerebri. In front the separation is complete, and it 

 appears to be behind also, but here the two hemispheres are attached to each other 

 over a small area by the pia mater. When the hemispheres are gently drawn 

 apart, it is seen that the fissure is interrupted in its middle part at a depth of a 

 little more than an inch (ca. 3 cm.) by a white commissural mass, the corpus 

 callosum ; this connects the hemispheres for about half of their length. The 

 transverse fissure separates the hemispheres from the cerebellum, and contains 

 the tentorium cerebelli. 



The convex or dorso-lateral surface' conforms closely to the cranial wall. 

 The medial or internal surface is flat and sagittal and bounds the longitudinal 

 fissure; to a large extent it is in contact with the falx cerebri, but behind the great 

 cerebral vein the two hemispheres are in contact and are attached to each other 

 over a small area as noted above. In well hardened specimens there is usually' an 

 impression for the vein in front of the area of adhesion. The base or ventral 

 surface (Fig. 499) is irregular. Its anterior two-thirds is adapted to the cerebral 

 fossa of the cranial floor. Crossing this area in front of the optic tract is a de- 

 pression, the transverse fossa (vallecula or fossa of Sylvius), which leads outward 

 to the lateral fissure (fissure of Sylvius), and lodges the middle cerebral artery. 

 In front of the fossa there is a considerable rounded elevation known as the 

 trigonum olfactorium. The trigonum and the inner part of the fossa are pierced 



^ In order to study the configuration of the hemisphere it should be separated from its fellow 

 by median section and from the brain stem by cutting across the mid-brain. Material for this 

 purpose should be hardened in situ. 



