114 MESENOEPHALON. 



according to Forel, it passes to another collection of nerve-cells, better marked in 

 most animals than in man, termed the interpeduncular ganglion (see p. 103). 



Snbthalamic tegmental region ; transitional region. The prolongation 

 of the tegmentum under the posterior part of the thalamus is divided by Forel into 

 three layers, which are named respectively from above down, the stratum dorsak, 

 the zona incerta, and the corpus subthalamicum or nucleus of Luys (fig. 83, 1, 2, 3). 

 The latter has here taken the place of the substantia nigra, lying next to the 

 prolongation of the crusta, the fibres of which are seen at the side of the subtha- 

 lamic tegmental region forming the internal capsule. The stratum dorsale consist 

 chiefly of fine longitudinal fibres prolonged from the posterior longitudinal bundle 

 according to Meynert, or from the fibres enclosing the tegmental nucleus according 

 to Forel, possibly from both sources. The red nucleus of the tegmentum is pro- 

 longed into its posterior part, and from this a considerable number of fibres stream 

 into the internal medullary lamina of the thalamus, and a well-marked bundle 

 passes across the internal capsule to the lenticular nucleus. Some fibres of this 

 layer, which are traceable downwards into the upper fillet (Wernicke), turn into the 

 external medullary lamina of the thalamus, which lies along the mesial side of the 

 internal capsule, and from here they probably diverge into the white matter of the 

 hemisphere ; whilst others, coursing through the zona incerta, and crossing the 

 inner capsule, join a tract (ansa lenticularis, fig. 84, a.L and p. 112) which leads to 

 the lenticular nucleus of the corpus stratum. Finally another bundle of fibres is 

 said to be traceable from the mesial nucleus of the thalamus backwards through the 

 subthalamic region into the dorsal part of the posterior commissure, and thus over 

 the Sylvian aqueduct into the tegmentum of the opposite side. 



The zona incerta is a reticular formation prolonged from that of the tegmentum ; 

 it passes anteriorly into the substantia interansalis. 



The corpus subthalamicum, or nucleus of Luys (figs. 83, 84, e.s.), is a well-marked 

 brown stratum of grey matter containing numerous nerve-cells, and a close plexus 

 of very fine medullated fibres. It is lens-shaped in section, and has an enclosing 

 envelope of white substance, through which strands of fine fibres pass from the 

 interior of the body mesially towards the zona incerta, and outwards and downwards 

 through the internal capsule. This stratum is distinct only in the Primates. 



The pineal body or gland (conarium, epiphysis cerebri) (fig. 82, Cn, fig. 91), 

 is a reddish body about the size of a small cherry-stone, and is named from its 

 supposed resemblance in shape to a fir-cone. It is connected with the posterior 

 part of the third ventricle, projecting backwards and downwards between the 

 superior pair of corpora quadrigemina. It is attached on each side by a broad 

 but flattened stalk of white fibres (pedunculus conarii) which is separated by the 

 pineal recess of the ventricle (p. 97) into a dorsal and ventral portion. The ventral 

 portion curves downwards ; it belongs to the ventral portion of the posterior 

 commissure. These fibres are said to - be derived from the optic tract near the 

 lateral geniculate body, and to go to the oculomotor nucleus of the opposite 

 side (compare p. 109). The upper portion extends on either side along the ridge- 

 like junction of the upper and mesial surfaces of the thalamus as the pineal 

 stria or tsenia fornicis (fig. 82, Tfo.}. At the sides the stalk merges into the 

 trigonum habenulse. The pia mater which invests the mesencephulon, covers the 

 pineal gland with a special investment before being prolonged as the velum inter- 

 positiim over the third ventricle and thalamus ; and the gland is liable to be torn 

 away in removing the pia mater. 



The pineal gland is composed of a number of hollow follicles generally spherical, 

 but in some cases tubular, separated from one another by ingrowths of connective 

 tissue. The follicles are almost filled with epithelial cells and often contain much 

 gritty calcareous matter (acervulus cerebri, brain-sand), composed of microscopic 



