506 THE NERVOUS SYSTEM. 
The corpus subthalamicum, 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. 
Pineal Body (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 hinder ends of the two thalamu, it occupies the depression on the dorsal aspect 
of the mesencephalon, which intervenes between the two superior quadrigeminal 
bodies. Its base, which is directed upwards, is attached by a hollow stalk or 
peduncle. This stalk is separated into a dorsal and a ventral part by the prolonga- 
tion backwards into it of a small pointed recess of the cavity of the third ventricle. 
The dorsal part of the stalk curves outwards and forwards, and on each optic 
thalamus becomes continuous with the tenia thalami and the subjacent stria 
medullaris; the ventral part is folded round a narrow but conspicuous, cord-lke 
band of white matter, which crosses the mesial plane immediately above the base 
of the pineal body and receives the name of the posterior commissure of the 
cerebrum. 
The pineal body is not composed of nervous elements. The only nerves in its midst are the 
sympathetic filaments which enter it, with its blood-vessels. It is composed of spherical and 
tubular follicles, filled with epithelial cells, and containing a variable amount of gritty, calcareous 
matter. 
The pineal body is a rudimentary structure, but in certain vertebrates it attains a much 
higher degree of development than in man. In the lizard, blind-worm, etc., it is present in the 
form of the so-called pineal eye. In structure it resembles, in these animals, an invertebrate eye, 
and it possesses a long stalk, in which nerve-fibres are dev eloped. Further, it is carried through 
an aperture in the cranial w all, and consequently lies close to the surface. 
Trigonum Habenule.—The sinall, triangular, depressed area which receives this 
name is placed immediately in front of the superior quadrigeminal body in the 
interval between the peduncle of the pineal body and the hinder end of the thalamus 
(Fig. 375, p. 502). It marks the position of an important collection of nerve-cells, 
which constitute the ganglion habenule. The axons of these cells are collected on 
the ventral aspect of the ganglion into a bundle, called the fasciculus retroflexus, 
which takes a curved course downwards and forwards in the tegmentum of the 
mesencephalon. The fasciculus retroflexus les close to the inner side of the red 
nucleus, and finally comes to an end in a group of cells termed the ganglion inter- 
pedunculare, situated in the lower part of the locus perforatus posticus (see p. 498). 
The ganglion habenule is likewise intimately connected with the stria medul- 
laris and the dorsal part of the stalk of the pineal body. 
As previously stated, the stria medullaris—a very evident band of white 
matter—lies on the optic thalamus, subjacent to the ependymal ridge termed the 
tenia thalami. When traced backwards, many of the fibres of the stria medullaris 
are observed to end amongst the cells of the ganglion habenule, whilst others are 
continued past the ganglion to enter the peduncle of the pineal body, and, through 
it, to reach the ganglion habenule of the opposite side, in connexion with the cells 
of which they terminate. The stria medullaris, therefore, ends partly in the 
ganglion habenule of its own side and partly in the corresponding ganglion of the 
opposite side. The decussation of fibres across the middle line forms the dorsal 
part of the pineal stalk or peduncle, and is frequently termed the commissura 
habenularum. 
When the stria medullaris is traced in the opposite direction, it is noticed to 
split into a dorsal and ventral part near the anterior pillar of the fornix. The 
dorsal part turns abruptly upwards, and, joining the fornix, is carried in it to the 
hippocampus major or cornu ammonis from cells in which its fibres take origin. 
The ventral part turns downwards and appears to spring from a collection of cells 
in the gray matter on the base of the brain close to the optic chiasma. The stria 
medullaris is believed to form a part of the olfactory apparatus. 
Commissura Posterior.—The posterior commissure is a slender band of white 
matter, which crosses the middle line under cover of the stalk of the pineal body 
and overlies the entrance of the aqueduct of Sylvius into the third ventricle. The 
connexions of this little band are not satisfactorily established, but Held believes 
