CHAPTER XV 



THE EXTERNAL CONFIGURATION OF THE CEREBRAL 

 HEMISPHERES 



Development. The cerebral hemispheres are formed by the evagination of 

 the alar laminae of the telencephalon, the rest of which remains as the boundary 

 of the rostral part of the third ventricle, and is known as the telencephalon 

 medium. The cavities of the evaginated portions are known as the lateral ven- 

 tricles and communicate with the third ventricle by way of the interventricu- 

 lar foramina (Figs. 15-17). Each of the cerebral hemispheres consists of two 

 ventrally placed portions, the rhinencephalon or olfactory lobe and corpus stria- 

 turn, and a third part, more extensive than the others, the pallium or primitive 

 cerebral cortex. The pallium expands more rapidly than the other parts, both 

 rostrally and caudally, and comes to overlie the diencephalon, from which it is 

 separated by the transverse fissure (Fig. 17). The fold of pia mater which is 

 inclosed within this fissure is known as the tela chorioidea; and from it a vascular 

 plexus grows into the lateral ventricle through the thin portion of the medial 

 wall of the hemisphere, where this is attached to the diencephalon. This forms 

 the chorioid plexus of the lateral ventricle and carries before it an epithelial cover- 

 ing from the ependymal lining, by which it is, in reality, excluded from the 

 ventricular cavity. This invagination of the medial wall of the hemisphere 

 produces the chorioid fissure. Ventrally the thickened part of the hemisphere, 

 known as the corpus striatum, remains in uninterrupted continuity with the 

 thalamus. 



At first the cerebral hemisphere has a relatively large cavity and thin walls. 

 As the pallium and ventricle enlarge they become bent around the thalamus 

 and corpus striatum (Fig. 17). The hemisphere becomes bean shaped and 

 the cavity curved. It expands rostrally to form the frontal lobe, caudally to 

 form the occipital lobe, and ventrolaterally to form the temporal lobe (Fig. 164). 

 Into each of these there is carried a prolongation of the lateral ventricle forming 

 respectively the anterior, posterior, and inferior horns. Between the temporal 

 and frontal lobes a deep fossa appears which is the forerunner of the lateral 

 fissure. At the bottom of this fossa is the insula, a portion of the cortex which 



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