THE TELENCEPHALON. 1193 



The Rhinencephalon. The rhinencephalon, using the term as including the various parts 

 of the hemisphere concerned with receiving and distributing the impulses of smell, comprises an 

 anterior division, the olfactory lobe, and the posterior or cortical division. The olfactory lobe 

 is suggested in embryos of the sixth week (Fig. 1029) by an elongated oval area, imperfectly 

 defined from the under surface ot the pallium by the rhinal furrow, and partially subdivided 

 by a faint transverse groove into a fore and a hind part. From the anterior division are 

 developed the olfactory bulb, tract, tubercle and striae and the parolfactory area ; from the 

 posterior, the anterior perforated space and the subcallosal gyrus. Although always relatively 

 rudimentary in man, the olfactory lobe at first contains a cavity prolonged from the lateral 

 ventricle, and in this respect resembles the corresponding but much larger olfactory lobe of 

 the osmatic animals which remains hollow. In the human brain, however, this cavity, the 

 olfactory ventricle, is only transient and later entirely disappears, its former position being 

 indicated in the adult structure by the central area of modified neurogliar tissue (page 1152). 



^\\Q posterior or cortical division includes the uncus, the hippocampus, the gyrus dentatus 

 with the associated supracallosal gray matter and nerve-strands. The original position of the 

 olfactory cortical area in the early human hemisphere corresponds with the permanent location 

 of the similar region in animals in which the expansion of the pallium never leads to the 

 formation of a well-marked occipito-temporal lobe. The early appearance of the primary 

 hippocampal and choroidal fissures defines an intervening tract upon the mesial surface of the 

 pallium. This is the primary gyrus dentatus and, with the hippocampal invagination, repre- 

 sents the earliest differentiation of the olfactory cortical area. Connection between the latter 

 and the region of the mammillary body is subsequently established by the advent of the 

 cortico-mammillary strand, later the chief part of the anterior column of the fornix. In 

 consequence of the migration of the hippocampus and the dentate gyrus incident to the for- 

 mation of the occipito-temporal regions of the hemisphere, the chief parts of the olfactory cortex 

 are carried downward and forward into the inferior horn of the lateral ventricle. Along with 

 the displaced cortical area necessarily follows the strand connecting it with the mammillary 

 region, hence the prolongation of the fornix, by means of its posterior pillar and the fimbria, 

 into the descending horn of the lateral ventricle. Although the major part of the olfactory 

 cortex thus comes to occupy the infero-mesial temporal region, a small portion retains its 

 superior connection and later, when the corpus callosum appears, becomes the greatly 

 attenuated sheet of gray matter which, with its reduced fibre-strands, overlies the upper 

 surface of the bridge as the atrophic supracallosal gyrus. 



The Corpus Striatum. The anlage of the corpus striatum, the fundamental ganglion of 

 the end-brain, is recognizable very early, and in brains of the fourth week appears as a triangu- 

 lar elevation between the cavity of the pallium and the optic recess (Fig. 912, ). Somewhat 

 later (Fig. 1030), this elevation, produced by a local thickening of the brain-wall, is seen pro- 

 jecting from the infero-lateral wall of- the pallium just in advance of the large foramen of Monro. 

 On the external surface of the pallium this thickening corresponds with the floor of the Sylvian 

 fossa ( Fig. 982) , and it is this close association between the corpus striatum and this area, 

 which fails to keep pace in its growth with the surrounding parts of the hemisphere, that leads 

 to its envelopment by the opercula and the permanent covering of the insula. The subsequent 

 partial separation of the corpus striatum into its two segments, the caudate and the lenticular 

 nucleus, as well as the isolation of a thin peripheral cortical plate, the claustrum, is effected 

 by the subsequent ingrowth of the strands of fibres which later become the internal and 

 external capsule. 



The Diencephalon. The posterior division of the fore-brain, the diencephalon, very early 

 (Fig. 1027) exhibits differentiation into an upper and a lower part. The former is the thalamen- 

 cephalon and the latter the pars mammillaris hypothalami, which correspond to expansions from 

 the dorsal and ventral laminae of the brain-tube respectively. The thalamencephalon is much 

 the larger and gives rise to the bulky mass of the thalamus from its anterior two-thirds and to 

 the epithalamus and the metathalamus from its posterior third. The epithalamus is prolonged 

 backward and from its upper surface an evagination occurs, the walls of which later thicken and 

 become the pineal body. Subsequent ingrowth of fibres across the bottom of a transverse 

 groove behind and below the pineal evagination leads to the establishment of the posterior 

 commissure, whilst thickening of the part of the epithalamus lying in front of the pineal recess 

 gives rise to the habenular region. The metathalamus appears at first as a triangular area lying 

 behind and to the outer side of the thalamus, with which it is closely connected. It early pre- 

 sents two slight external elevations which become the lateral, and median geniculate bodies. 

 The diencephalic division of the hypothalamus early shows a differentiation into a series of eleva- 

 tions and furrows, the thickened areas becoming the mammillary body and the subthalamic 

 region. 



The roof of the diencephalon is thin from the first and remains so. In front it is directly 

 continuous with the correspondingly attenuated plate which connects the hemispheres and, 

 arching over the foramen of Monro, joins the lamina terminalis that closes the cavity of the 



