THE CEREBRAL HEMISPHERES. 157 



a. External form (Figs. 98, 102, 103 He and L.ot). The two 

 cerebral hemispheres form together the largest section of the 

 brain ; from above they are seen to be separated by a dorsal lon- 

 g-it udinal fissure, which is here well marked : each hemisphere is an 

 ovoid body with the smaller end directed forwards and continuous 

 with the corresponding olfactory lobe ; the 

 posterior end forms one half of the anterior 

 boundary of the thalamencephalon. The 

 olfactory lobes are two elongated, rounded 

 bodies directly continuous with the cor- 

 responding cerebral hemispheres, and like- 

 wise partially separated in the median line 

 by a dorsal longitudinal fissure: at the section through the lower division 

 point of union of the cerebral hemispheres of the pi^tao body (magnified 



. . 360 times) ; from Stieda. 



and olfactory lobes is a faintly marked a Tubes lined with epithelium. 



j b Blood-Teasels. 



transverse depression. 



On the ventral surface the parts are again marked off from 

 one another by a corresponding ventral longitudinal fissure and a 

 transverse groove; the two longitudinal fissures being continuous, 

 anteriorly, between the olfactory lobes. The cerebral hemispheres 

 appear to be more widely separated behind (Fig. 102) than is 

 the case on the dorsal surface, and in the space so formed is 

 the lamina terminaUs (Lt}. Seen from the side, the slight depres- 

 sion of the upper surface, between the cerebral hemispheres and 

 the olfactory lobes, is seen to be continued downwards and 

 slightly backwards to join the corresponding groove on the inferior 

 surface. 



The longitudinal fissures are shallow and do not meet, except 

 at one point, at about the middle of the cerebral hemispheres 

 (Fig. 105). The olfactory bulbs arise superficially (Figs. 102, 103 /' 

 and L.olJ from the whole length of the olfactory lobe, between the 

 anterior extremity (7') and the posterior (L.ofy, where they are also 

 attached to the cerebral hemispheres. 



/3. Internal structure. The cerebral hemispheres and olfactory 

 lobes are hollow, the common cavity of each side being known 

 as the lateral ventricle (Tenti-indit* laterali*} these ventricles 

 communicate with each other, and with the third ventricle 

 (Fig. 105). The narrow aperture by which the lateral ventricles 

 communicate is known as the Foramen of Monro (HF)-, it 

 communicates with the third ventricle posteriorly, and with the 

 space between the cerebral hemispheres anteriorly, and thus forms 



