140 THE CEREBRUM. 



and projects backward over the mid-brain. Laterally, it rests 

 against the superior lamina of the internal capsule, which separates 

 it from the lentiform nucleus. The thalamus is shaped like an 

 egg, with the small end directed forward. It has an anterior and 

 a posterior extremity and four surfaces: Superior, inferior, 

 medial and lateral. 



Extremities. The anterior extremity of the thalamus is lost 

 in a large group of fibers (frontal stalk) which runs through the 

 frontal part of the internal capsule. The posterior end (Fig. 44) 

 presents a large pillow-like prominence, the pulvinar, and beneath 

 it, are two smaller swellings; the outer one, which forms the lowest 

 point of the thalamus, is the lateral geniculate body; the medial 

 geniculate body is the other. The two geniculate bodies constitute 

 the metathalamus (Fig. 43). 



Surfaces. The medial surface of the thalamus forms the 

 immediate lateral wall of the third ventricle as far down as the 

 sulcus hypothalamicus (Fig. 28). It is joined to the internal 

 surface of the opposite thalamus by the massa intermedia. It is 

 bounded above by the medullary stria. The superior surface is 

 composed of a thin lamina of longitudinal white fibers, derived 

 from the optic tract and radiation, called the stratum zonale. 

 The superior surface of the thalamus is divided by an oblique 

 groove, the chorioidal groove, corresponding in position to the 

 border of the fornix, into two areas a medial and lateral (Fig. 

 44). The medial area is covered by the chorioid tela of the third 

 ventricle and the fornix. Internally, it is bounded by the medul- 

 lary stria of the thalamus. Posteriorly, next the stria, is a tri- 

 angular depression bounded, behind, by a transverse groove in 

 front of the corpora quadrigemina, and by a slight groove, the 

 sulcus habenulae, externally. That depressed surface is called 

 the triangle of the habenula (trigonum habenulae). Beneath the 

 triangle is one of the thalamic nuclei, the nucleus habenula. The 

 lateral area of the superior surface is seen in the floor of the lateral 

 ventricle. It presents an anterior elevation, the anterior tubercle 

 (tuberculum anterior thalami}, beneath which is the anterior 

 nucleus of the thalamus. The chorioid sheet of epithelium, 

 extending from the fornix to the stria terminalis, covers this outer 



