Til K ENCEPHALON 777 



Laterally 1 ho temporal lobes, with their rounded anterior cxt remit ies. the tem- 

 poral poles, are each separatee I from the frontal and parietal lobes above !>y the 

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

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



The Miri'are of earh 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 and 

 in their deirrees of curvature. The larger and deeper of them are similar in the t\\o 

 hemispheres; most of them are individually variable, but each gyms of one hemi- 

 sphere 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 com- 

 missunil pathway bet \\een the cerebral hemispheres, may be seen. The occipital 

 margin of this large transverse band of white substance is rounded and thickened 

 into (lie splenium of the corpus callosum, while its frontal margin is curved ventrally 

 into the genu. 



The base of the encephalon is more irregular than the convex surface, and con- 

 sists 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 

 culloxtim. the downward continuation of the curve of the genu, is exposed. 



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

 the superior wall of the orbit. The orbital gyri with their respective in-ln'tnl sulci 

 occupy this concave area. 



Trie 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. Kach olfactory bulb is the first central connec- 

 tion or 'nucleus of reception ' of the olfactory nerve, the first of the twelve cranial 

 nerves. A few fine filaments of this nerve may often lie discerned penetrating the 

 ventral surface of the bulb. They are non-medullatcd. The olfactory bulb and tract 

 lies in the olfurtori/ xulois, which forms the lateral boundary of the <j;/rus rectus, the 

 most mesial gyms of the inferior surface of the frontal lobe. 1 pon reaching the 

 area of Broca (area parolfactoria), or the region about the inner extremity of the 

 gyms rectus, each olfactory tract undergix's a slight expansion, the olfactory tuber- 

 cle, and then divides into three roots or olfactory striae a medial, an intermediate, 

 and a lateral, which comprise the olfactory trigone. The strue begin their respec- 

 tive 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 basis encephali in which the substance of the brain becomes continuous across 

 the mid-line. 



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

 (the second pair of cranial nerves) <?l>me together and fuse to form the optic chiasma. 

 Thence the optic tracts disappear under the poles of the temporal lobes in their back- 

 ward course to the thnlnmi and rnrjiiini iji'iiiciilntn 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, the infundibulum, with the hypophysis or pituitary 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 albic.-mtial. each of which is the superficial 

 evidence of the fornix. one of the larger association fasciculi of the cerebrum. The 

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

 the cerebral hemispheres with all the structures below them. They diverge from the 

 anterior border of thepons (Yarolii) and. one for each hemisphere, disappear under 

 the poles of the temporal lobes. The pons (brachium pontis or middle cerebellar 

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

 hemispheres. 



The oculomotor or third pair of cranial nerves make their exit from the poste- 

 rior perforated substance in the interpeduncular fossa just behind the corpora 

 mammillaria. 



The trochlear nerves, or the fourth pair, emerge around the lateral aspects of 

 the pedunculi cerebri along the anterior border of the pons. The fourth is the 



