MAIN SUB-DIVISIONS 355 



missure; (3) by a thin membrane called the lamina terminalis, 

 through which the fibres of the anterior commissure run 

 (Figs. 136, 156, 159, 173). 



The two hemispheres constitute, together, the telencephalon, 

 which is the last formed, but the most highly developed, 

 portion of the brain. Each hemisphere contains a cavity 

 called the lateral ventricle, and there are, therefore, in the 

 telencephalon two lateral ventricles, a right and a left, known 

 also, though less commonly, as the first and the second 

 (Figs. 163, 164, 165). 



Immediately below and between the two cerebral hemi- 

 spheres lies a portion of the brain called the diencephalon. 

 It is continuous, posteriorly, with the mesencephalon or mid- 

 brain, and, anteriorly and laterally, with the cerebral hemi- 

 spheres. In the interior of the diencephalon there is a cavity 

 called the third ventricle (Figs. 136, 168). The cavity is 

 continuous, anteriorly, through apertures called the inter- 

 ventricular foramina, with the lateral ventricles of the telen- 

 cephalon, and, posteriorly, with a canal, called the aquaductus 

 cerebri, which runs through the mid-brain and connects the 

 cavity of the diencephalon with that of the rhomb encephalon 

 or hind-brain. 



When examined from its ventricular side, each half of the 

 diencephalon is seen to be separated into two parts, a dorsal 

 and a ventral, by an antero-posterior sulcus called the sulcus 

 hypothalamicus ; the dorsal part is called the thalamus, the 

 ventral part is the hypothalamus. In Fig. 136 the point and 

 the adjacent part of the arrow lie in the hypothalamic 

 sulcus. 



The dorsal wall of the. cavity of the diencephalon is called 

 the roof of the third ventricle, and from it a conical mass, called 

 the pineal body, projects backwards over the mid-brain ; it 

 forms part of the epithalamus. The remainder of the epi- 

 thalamus lies anterior and lateral to the pineal body, on the 

 upper and posterior part of the thalamus, and it consists of the 

 habenula and the trigonum habenulcz, on each side (Fig. 176). 



Forming part of the ventral wall of the diencephalon are 

 two round, white bodies, called the corpora mamillaria, and 

 further forwards is a conical projection called the tuber 

 cinereum, which is connected with the hypophysis by a thin 

 stalk, called the infundibulum (Figs. 136, 139). All the parts 

 of the ventral wall are parts of the hypothalamus. 

 in 23 a- 



