794 THE NERVOUS SYSTEM 



Viewed from above, the cerebrum comprises almost the entire dorsal aspect, the 

 occipital lobes overlapping the cerebellum to such an extent that only the lateral 

 and lower margins of the cerebellar hemispheres are visible. The great longitu- 

 dinal fissure of the cerebrum separates the cerebral hemispheres, 



Laterall}' the temporal lobes, with their rounded anterior extremities, the tem- 

 poral poles, are each separated from the frontal and parietal lobes above by the 

 lateral cerebral fissure (fissure of Sylvius). In the depths of this fissure and over- 

 lapped b}^ the temporal lobe is situated the insula, or island of Reil (central lobe). 



The surface of each cerebral hemisphere is thrown into numerous folds or 

 curved elevations, the gyri cerebri or convolutions, which are separated from each 

 other by slit-like fissures, the sulci cerebri. The gyri (and sulci) vary greatly in 

 length, in depth, and in their degrees of curvature. The larger and deeper of them 

 are similar in the two hemispheres; most of them are individually variable, but 

 each gyrus of one hemisphere is homologous with that of the like region of the 

 other hemisphere. By gently pressing open the great longitudinal fissure, the 

 corpus callosum, the chief commissural pathway between the cerebral hemi- 

 spheres, may be seen. The occipital margin of this large transverse band of white 

 substance is rounded and thickened into the splenium of the corpus callosum, 

 while its frontal margin is curved ventrally into its genu and rostrum. 



The base of the encephalon (fig. 625) is more irregular than the convex surface, 

 and consists of a greater variety of structures. In the mid-line between the frontal 

 lobes appears the anterior and inferior extension of the great longitudinal fissure. 

 When the margins of this are separated, the outer aspect of the rostrum of the 

 corpus callosum, the downward continuation of the curve of the genu, is exposed. 



The inferior surface of each frontal lobe is concave, due to its compression upon 

 the superior wall of the orbit. The orbital gyri with their respective orbital sulci 

 occupy this concave area. 



The cranial nerves [nervi cerebrales]. — Along the mesial border of each orbital 

 area, and parallel with the great longitudinal fissure, lie the olfactory bulbs con- 

 tinued into the olfactory tracts. Each olfactory bulb is the first central connection 

 or the 'nucleus of reception' of the olfactory nerve, the first of the cranial nerves. 

 A few fine filaments of this nerve may often be discerned penetrating the ventral 

 surface of the bulb. The olfactory bulb and tract lies in the olfactory sulcus, 

 which forms the lateral boundary of the gyrus rectus, the most mesial gyrus of the 

 inferior surface of the frontal lobe. Upon reaching the area of Broca (area parol- 

 factoria), or the region about the medial extremity of the gyrus rectus, each 

 olfactory tract undergoes a slight expansion, the olfactory tubercle, and then 

 divides into three roots or olfactory striae — a medial, an intermediate, and a lateral, 

 which comprise the olfactory trigone. The striae begin their respective courses 

 upon the anterior perforated substance, an area which contains numerous small 

 foramina through which the antero-lateral group of central cerebral arteries enters 

 the brain. This region forms the anterior boundary of that area of the base of 

 the encephalon in which the substance of the brain becomes continuous across the 

 mid-line. 



At the medial boundary of the anterior perforated substance the optic nerves 

 come together and fuse to form the optic chiasma. Thence the optic tracts dis- 

 appear under the poles of the temporal lobes in their backward course to the thai- 

 ami and the geniculate bodies or metathalami. 



Immediately behind the optic chiasma occurs that diverticulum from the floor 

 of the third ventricle known as the tuber cinereum. It is connected by its tubular 

 stalk, th(! infundibulum, with the hypophysis or i)ituitary body, which occupies 

 its special depression (sella turcica) in the floor of the cranium and is usually torn 

 from the encephalon in the process of its removal. Behind the tuber cinereum 

 are the two mammillary bodies (corpora albicantia), each of which is connected 

 with the fornix, one of the larger association fasciculi of the cerebrum. The 

 peduncles of the cerebrum (crura cerebri) are the two great funiculi which asso- 

 ciate the ccreljral hemispheres with all the structures below them. They diverge 

 from the anterior border of the pons (Varoli) and, one for each hemisphere, dis- 

 appear und(T the poles of the temporal lobes. The pons (brachium pontis or 

 middle cerebellar peduncle) is chiefly a bridge of white substance or a commissure 

 between the cerebeUar hemispheres. 



