DEVELOPMENT OF THE NERVOUS SYSTEM 



745 



in the adult brain are represented by the lateral root of the olfactory tract and the 

 uncus. The position and connections of the remaining portions of the rhinen- 

 cephalon are described with the anatomy of the brain. 



The corpus striatum (Figs. 651 and 653) appears in the fourth week as a triangular 

 thickening of the floor of the telencephalon between the optic recess and the 

 interventricular foramen, and continuous behind with the thalamic part of the 

 diencephalon. It increases in size, and by the second month is seen as a swelling 

 in the floor of the future lateral ventricle ; this swelling reaches as far as the posterior 

 end of the primitive hemisphere, and when this part of the hemisphere grows 

 backward and downward to form the temporal lobe, the posterior part of the corpus 

 striatum is carried into the roof of the inferior horn of the ventricle, where it is 

 seen as the tail of the caudate nucleus in the adult brain. During the fourth and 

 fifth months the corpus striatum becomes incompletely subdivided by the fibers of 

 the internal capsule into two masses, an inner, the caudate nucleus, and an outer, 

 the lentiform nucleus. In front, the corpus striatum is continuous with the anterior 

 perforated substance; laterally it is confluent for a time with that portion of the 

 wall of the vesicle which is developed into the insula, but this continuity is sub- 

 sequently interrupted by the fibers of the external capsule. 



Falx cerebri 





\k ' Edge of white substance 

 \Edge of grey cortical 

 unbalance 

 Jlippocampal fissure 



Cs. Corpus striatum. Th. Thalamus. 



FIG. 656. Diagrammatic coronal section of brain to show relations of neopallium. (After His.) Cs. Corpus striatum. 



Th. Thalamus. 



The neopallium (Fig. 656) forms the remaining, and by far the greater, part of the 

 cerebral hemisphere. It consists, at an early stage, of a relatively large, more or 

 less hemispherical cavity the primitive lateral ventricle enclosed by a thin wall 

 from which the cortex of the hemisphere is developed. The vesicle expands in all 

 directions, but more especially upward and backward, so that by the third month 

 the hemispheres cover the diencephalon, by the sixth they overlap the mid-brain, 

 and by the eighth the hind-brain. 



The median lamina uniting the two hemispheres does not share in their expan- 

 sion, and thus the hemispheres are separated by a deep cleft, the forerunner of 

 the longitudinal fissure, and this cleft is occupied by a septum of mesodermal 

 tissue which constitutes the primitive falx cerebri.' Coincidently with the expan- 

 sion of the vesicle, its cavity is drawn out into three prolongations which represent 



