PAKTS DEEIVED FKOM THE DIENCEPHALON. 615 



As previously stated, -the stria medullaris a very evident band of white 

 matter lies on the thalamus, subjacent to the ependymal ridge termed the tsenia 

 thalami. When traced backwards, many of the fibres of the stria medullaris 

 are observed to end amongst the cells of the ganglion habenulse, whilst others are 

 continued past the ganglion to enter the peduncle of the pineal body, and, through 

 it, to reach the ganglion habenulae of the opposite side, in connexion with the cells 

 of which they terminate. The stria medullaris, therefore, ends partly in the 

 ganglion habenulas of their own side and partly in the corresponding ganglion of the 

 opposite side. The decussation of fibres across the median plane forms the dorsal 

 part of the pineal stalk or peduncle, and is termed the commissura habenularum. 



When the stria medullaris is traced in the opposite direction, it is noticed 

 to split into dorsal and ventral parts near the column of the fornix. The dorsal 

 part arises from cells in the hippocampus : these fibres pass into the fornix and 

 when they reach its column they turn abruptly backwards to enter the stria 

 medullaris. The ventral part springs from a collection of cells in the gray matter 

 on the base of the brain close to the optic chiasma. The striae medullares are 

 believed to form a part of the olfactory apparatus. 



Gommissura Posterior. The posterior commissure is a slender band of white 

 matter, which crosses the median plane under cover of the stalk of the pineal body 

 and overlies the entrance of the aqueduct of the brain into the third ventricle. The 

 fibres which enter into the formation of the posterior commissure are believed to 

 arise in a special nucleus, which is placed in the central gray matter immediately 

 above the oculo-motor nucleus. They decussate with each other across the median 

 plane and thus the commissure is formed. The other connexions of this little 

 band are not satisfactorily established, but Held believes that some of its ventral 

 fibres pass downwards into the medial longitudinal bundle. 



Substantia Perforata Posterior. This has already been described on p. 542. 

 Some delicate bands of white matter, termed the tsenia pontis, may frequently be 

 seen emerging from the gray matter of this region ; they then curve round the 

 pedunculus cerebri in close relation to the superior border of the pons, with which 

 they enter the cerebellum to end in the nucleus dentatus (Horsley). 



Corpora Mamillaria. The corpora mamillaria are two round white bodies, 

 each about the size of a pea, which lie side by side in the interpeduncular fossa on 



base of the brain, immediately in front of the substantia perforata posterior. 



Each corpus mamillare is coated on the outside by white matter derived 

 the column of the fornix, and contains, in its interior, a composite gray 



ileus with numerous nerve-cells. Several important strands of fibres are con- 

 nected with the corpus mamillare: (1) The column of the fornix curves' down- 

 wards in the side wall of the third ventricle to reach the corpus mamillare, 

 and their fibres end amidst the cells of that body. (2) A bundle of fibres, the 

 fasciculus mamillo-thalamicus, takes origin in the midst of each corpus mamillare 

 and extends upwards into the thalamus, to end in fine arborisations around the 

 large cells in the anterior thalamic nucleus. (3) Another bundle of fibres, the 

 pedunculus corporis mamillaris, takes form within the corpus mamillare and extends 

 downwards in the gray matter of the floor of the third ventricle, to reach the 

 tegmentum of the mesencephalon. These tracts, together with the strise medullares 

 (thalami) and the fasciculi retroflexi, are amongst the most ancient fibre-systems in 

 the brain. They represent the paths by which olfactory impulses may reach the 

 brain-stem, and perhaps the spinal medulla also, and so influence the muscles of 

 the body. 



Tuber Cinereum and Infundibulum. The tuber cinereum is a small, slightly 

 prominent field of gray matter, which occupies the anterior part of the inter- 

 peduncular fossa between the corpora mamillaria behind and the optic chiasma 

 in front. From its anterior part the infundibulum, or stalk of the hypophysis, 

 projects downwards and connects the hypophysis with the base of the brain. In its 

 upper part the infundibulum is hollow, a small, funnel-shaped diverticulum of the 

 a-vity of the third ventricle being prolonged downwards into it. 



Hypophysis (O.T. Pituitary Body). This is a small oval structure, flattened 

 above downwards, and with its long axis directed transversely, which 



40 c 



