iii8 HUMAN ANATOMY. 



The dorsal zones of the lateral wall of the mid-brain give rise to the quadrigeminal plate, 

 whose external surface is at first smooth but later marked by a temporary median longitudinal 

 ridge. About the third fcetal month, with the exception of its lower end, which persists as the 

 frenulum veli, this ridge is succeeded by a longitudinal groove bounded on either side by an 

 elevation. The elevations of the two sides mark the appearance of the corpora bigemina, cor- 

 responding to the optic lobes of the lower vertebrates. During the fifth month, an obliquely 

 transverse furrow forms on each side, by which the paired elevations are subdivided into four 

 eminences, the corpora quadrigemina. About this time the corpora geniculata, which however 

 belong developmentally to the diencephalon, are also differentiated and for awhile are rela- 

 tively very large and prominent. 



The ventral zones greatly thicken and give origin to the tegmentimi, including the nuclei 

 of the oculomotor and of the trochlear nerves and, perhaps, the red nuclei, and the mantle layer 

 of the cerebral peduncles with the interpeduncular substance. The floor-plate becomes com- 

 pressed between the expanding ventral zones of the lateral walls and probably is represented 

 by the raphe. Since the fibre-systems of the crustae are, for the most part, derived from sources 

 outside the brain-stem, their appearance within the peduncles follows a secondary ingrowth, 

 and only after such invasion do the cerebral crura present their characteristic ventral prom- 

 inence. The cortico-pontile tracts share with the pyramidial fibres the characteristic of tardy 

 myelination, since they do not acquire their medullary coat until some time after birth. Among 

 the earliest of the cortico-bulbar fibres to become medullated (a few weeks after birth) are those 

 destined for the motor cranial nerves by way of the crustal or pyramidal fillet of Flechsig. 

 According to Kolliker, the stratum intermedium, which is closely related to the substantia 

 nigra, not only in position but also by the destination of many of its fibres, contains a consider- 

 able number of medullated fibres by the ninth foetal month. 



THE FORE-BRAIN. 



It will be recalled that the fore-brain, the anterior primary cerebral vesicle, gives 

 rise to two subdivisions, the telencephalon and the diencephalon (page 1060). Since 

 the latter lies immediately in front of the mid-brain, in following the order in which 

 the brain-segments have been described, the diencephalon next claims attention. 



THE DIENCEPHALON. 



Strictly considered upon the basis of the classic subdivision suggested by His, the 

 diencephalon, or inter-brain, includes (i) a large dorsal portion, the thalamen- 

 cephalon and (2) a small ventral portion, the pars mammillaris hypothalami, 

 together with (3) the enclosed remains of the posterior part of the cavity of the 

 fore-brain, as represented by the greater part of the third ve^itricle. The thalamen- 

 cephalon, in turn, includes : (a) the thalamus, {b) the epithalamus, comprising the 

 pineal body, the habenular region and the posterior commissure, and (r) the meta- 

 thalamus, including the corpora geniculata. Since, however, the description of the 

 third ventricle and its surrounding structures — the essential features of this segment 

 of the adult brain — requires the inclusion of parts belonging to the telencephalon 

 (pars optica hypothalami), it will be more convenient to disregard their strict 

 developmental relations and include the representatives of the pars optica in the 

 consideration of the diencephalon. 



The Thalamus. — After removal of the overlying structures — the corpus callo- 

 sum, the fornix and the velum interpositum — the thalami (thalarai), also called 

 the optic thalami, are seen as two conspicuous masses of gray matter separated by a 

 narrow cleft, the third ventricle. Each thalamus is an ovoid ganglionic mass, blunt 

 wedge-shaped, as seen in cross-sections (Fig. 967), whose long axis extends from 

 the narrow anterior pole backward and outward. Of its four surfaces, the lateral and 

 ventral are blended with the surrounding nervous tissue, and the mesial and dorsal 

 are to a large extent free. The large superior surface is irregularly triangular 

 in outline, slightly convex in the frontal plane and markedly so in the sagittal, and 

 covered with a thin layer of nerve-fibres, the stratum zonale, which imparts a 

 ' whitish color. This stratum is composed of fibres which are traceable on the one 

 hand to the optic tract, and on the other to the optic radiation in the hind part of 

 the internal capsule. Laterally, the superior surface is separated from the caudate 

 nucleus by a groove which obliquelv crosses the floor of the lateral ventricle and 

 lodges a narrow band of fibres, the taenia semicircularis (stria terniinalis) and, in 

 its anterior part, the vein of the corpus striatum. In its front half, where it bounds the 



