812 NEUROLOGY 





The superior colliculus, the pulvinar, and the lateral geniculate body receive 

 many fibers from the optic tracts, and are therefore intimately connected with 

 sight, constituting what are termed the lower visual centers. Extirpation of the 

 eyes in a newly born animal entails an arrest of the development of these centers, 

 but has no effect on the medial geniculate bodies or on the inferior colliculi. More- 

 over, the latter are well-developed in the mole, an animal in which the superior 

 colliculi are rudimentary. 



The Epithalamus comprises the trigonum habenulae, the pineal body, and the 

 posterior commissure. 



The trigonum habenulae is a small depressed triangular area situated in front 

 of the superior colliculus and on the lateral aspect of the posterior part of the tsenia 

 thalami. It contains a group of nerve cells termed the ganglion habenulae. Fibers 

 enter it from the stalk of the pineal body, and others, forming what is termed the 

 habenular commissure, pass across the middle line to the corresponding ganglion 

 of the opposite side. Most of its fibers are, however, directed downward and form 

 a bundle, the fasciculus retroflexus of Meynert, which passes medial to the red 

 nucleus, and, after decussating with the corresponding fasciculus of the opposite 

 side, ends in the interpeduncular ganglion. 



The pineal body (corpus pineale; epiphysis) is a small, conical, reddish-gray body 

 which lies in the depression between the superior colliculi. It is placed beneath the 

 splenium of the corpus callosum, but is separated from this by the tela chorioidea 

 of the third ventricle, the lower layer of which envelops it. It measures about 

 8 mm. in length, and its base, directed forward, is attached by a stalk or peduncle 

 of white substance. The stalk of the pineal body divides anteriorly into two 

 laminae, a dorsal and a ventral, separated from one another by the pineal recess 

 of the third ventricle. The ventral lamina is continuous with the posterior com- 

 missure; the dorsal lamina is continuous with the habenular commissure and 

 divides into two strands the medullary striae, which run forward, one on either 

 side, along the junction of the medial and upper surfaces of the thalamus to blend 

 in front with the columns of the fornix. 



The posterior commissure is a rounded band of white fibers crossing the middle 

 line on the dorsal aspect of the upper end of the cerebral aqueduct. Its fibers 

 acquire their medullary sheaths early, but their connections have not been definitely 

 determined. Most of them have their origin in a nucleus, the nucleus of the poste- 

 rior commissure (nucleus of Darkschewitsch) , which lies in the central gray substance 

 of the upper end of the cerebral aqueduct, in front of the nucleus of the oculomotor 

 nerve. Some are probably derived from the posterior part of the thalamus and from 

 the superior colliculus, while others are believed to be continued downward into 

 the medial longitudinal fasciculus. 



The Hypothalamus (Fig. 720) includes the subthalamic tegmental region and 

 the structures forming the greater part of the floor of the third ventricle^ viz., the 

 corpora mammillaria, tuber cinereum, infundibulum, hypophysis, and optic chiasma. 



The subthalamic tegmental region consists of the upward continuation of the 

 tegmentum; it lies on the ventro-lateral aspect of the thalamus and separates 

 it from the fibers of the internal capsule. The red nucleus and the substantia 

 nigra are prolonged into its lower part; in front it is continuous with the substantia 

 innominata of Meynert, medially with the gray substance of the floor of -the third 

 ventricle. 



It consists from above downward of three strata: (1) stratum dorsale, directly 

 applied to the under surface of the thalamus and consisting of fine longitudinal 

 fibers; (2) zona incerta, a continuation forward of the formatio reticularis of the 

 tegmentum; and (3) the corpus subthalamicum (nucleus of Luys), a brownish mass 

 presenting a lenticular shape on transverse section, and situated on the dorsal 

 aspect of the fibers of the base of the cerebral peduncle; it is encapsuled by a lamina 



