614 THE NEKVOUS SYSTEM. 



by the nucleus, a very evident and dense mass of fibres. The fibres of the medial 

 lemniscus, of the brachium conjunctivum, and of the red nucleus are prolonged 

 upwards into the ventral part of the thalamus, where they end in connexion with 

 the thalamic cells (ventro-lateral nucleus). 



The substantia nigra is likewise carried into the hypothalamic region, where 

 it maintains its original position on the dorsal aspect of the basis pedunculi. 

 As it is traced upwards, it is seen gradually to diminish in amount. It shrinks 

 from the medial to the lateral side, and finally disappears when the posterior part 

 of the corpus mamillare is reached. 



In frontal sections through the hypothalamic region, the most conspicuous object 

 which comes into view is the nucleus hypothalamicus or the nucleus of Luys (Fig. 541). 

 It is a small mass of gray matter, shaped like a biconvex lens, which makes its appear- 

 ance on the dorsal aspect of the basis pedunculi immediately to the lateral side of 

 the substantia nigra. At first it lies in an angle which is formed by the meeting 

 of the cerebral peduncle and the internal capsule; but, rapidly enlarging in a 

 medial direction, it takes the place of the diminishing substantia nigra on the dorsal 

 surface of the basis pedunculi at the level of the inferior part of the corpus mamillare. 

 The nucleus hypothalamicus is rendered all the more evident by the fact that it is 

 sharply defined by a thin capsule of white fibres. On its medial aspect these fibres 

 proceed medially and form a very evident decussation across the median plane in the 

 floor of the third ventricle, immediately above the posterior ends of the corpora 

 mamillaria. 



The nucleus hypothalamicus, in the fresh condition, presents a brownish colour, 

 partly from the fact that its cells are pigmented, and partly also on account of 

 the numerous capillary blood-vessels which pervade its substance. 



Corpus Pineale. This is a small, dark, reddish body, about the size of a cherry 

 stone and shaped after the fashion of a fir-cone. Placed between the posterio 

 ends of the two thalami, it occupies the depression on the dorsal aspect of th 

 mesencephalon, which intervenes between the two superior colliculi. Its base 

 which is directed upwards, is attached by a hollow stalk or peduncle. Thi 

 stalk is separated into a dorsal and a ventral part by the prolongation back 

 wards into it of a small pointed recess of the cavity of the -third ventricle. Th 

 dorsal part of the stalk curves laterally and forwards, and, on each thalamus 

 becomes continuous with the tsenia thalami ; the ventral part is folded round 

 narrow but conspicuous cord-like band of white matter, which crosses the medial 

 plane immediately below the base of the pineal body and receives the name of th 

 posterior commissure of the cerebrum (Fig. 519, p. 585). 



The pineal body is not composed of nervous elements. The only nerves in its midst are th 

 sympathetic filaments which enter it, with its blood-vessels. It is composed of spherical an 

 tubular follicles, filled with epithelial cells, and containing a variable amount of gritty, calcareou 

 matter. 



The pineal body is a rudimentary structure, but in certain vertebrates it attains a inuc 

 higher degree of development than in man. In the lamprey, lizard, etc., it is present in th 

 form of the so-called pineal eye. In structure it resembles, in these animals, an invertebrate ey( 

 and it possesses a long stalk, in which nerve-fibres are developed. Further, it is carried throng 

 an aperture in the cranial wall, and consequently lies close to the surface on the dorsum of tn 

 head between the parietal bones. 



Trigonum Habenulae. The small, triangular, depressed area which receive 

 this name is placed immediately in front of the superior colliculus in the intern 

 between the peduncle of the pineal body and the posterior end of the thalami 

 (Fig. 539, p. 610). It marks the position of an important collection of nerve-cell 

 which constitute the ganglion habenulae. The axons of these cells are collected o 

 the ventral aspect of the ganglion into a bundle, called the fasciculus retroflexu 

 which takes a curved course downwards and forwards in the tegmentum of tl 

 mesencephalon. The fasciculus retroflexus lies close to the medial side of tl 

 red nucleus, and finally comes to an end in a group of cells termed the gangli( 

 interpedunculare, situated in the inferior part of the substantia perforata poster! 

 (see p. 591). 



The ganglion habenulee is likewise intimately connected with the stria medullai 

 (tsenia thalami) and the dorsal part of the stalk of the pineal body. 



