THE BRAIN OR ENCEPHALON 851 



General Appearance and Topography of the Brain. Corresponding to the 

 varieties of cranial form, the shape of the fresh or the successfully preserved brain 

 varies from the ovoid to the nearly spherical form, as viewed dorsally. The 

 frontal pole is usually narrower, though more squarely formed; while the parieto- 

 occipital portion is more massive, but more sharply pointed in each half. The 

 outline is often rather that of an irregular pentagon with its angles rounded off. 

 A dorsal view shows only the extensive convex surface of the tw T o great convoluted 

 cerebral hemispheres (hemicerebra) separated by a median cleft, the intercerebral 

 fissure (Jissura longitudinales cerebri). On divaricating the cerebral halves it is 

 seen that the separation is not a total one, for in the depths of the fissure a broad 

 commissural mass of white fibres the corpus callosum joins the cerebral hemi- 

 spheres. Frontad the longitudinal fissure is continued to the ventral or basal 

 aspect of the brain; caudad it passes into the fissura transversa cerebri (tentorial 

 hiatus) or interval, separating the cerebrum from the cerebellum. 



In a lateral view the continuity of the spinal cord with the medulla oblongata, 

 then the pons and cerebellum are seen in part, overlapped by the cerebrum. 

 Prominent is the temporal lobe with its rounded pole, separated from the frontal 

 and parietal lobes by a deep cleft, the sylvian fissure, in whose depths overlapped 

 by the opercula of the adjacent lobes lies the island of Reil or insula. 



A ventral view presents many of the subdivisions of the brain. Here is seen 

 the continuity of the spinal cord, with the short and slightly expanding medulla 

 oblongata lying ventrad of the cerebellum and somewhat buried in its vallecula 

 or depression between the lateral hemispheres, which alone are visible. The 

 cerebellum is a grayish-colored mass of considerable size and easily recog- 

 nized by its foliated appearance, due to the numerous parallel and closely set 

 curved fissures. A mass of white fibres, the pons, passes transversely from one 

 cerebellar hemisphere to the other, ventrad of the upper portion of the medulla 

 oblongata. Above the pons are seen two large bundles, the crura cerebri, one on 

 either side, diverging to pass into the cerebral halves. The interval between the 

 divergent crura cerebri and temporal poles laterad and the orbital portions of 

 the cerebrum frontad contains a number of important structures. Encircling 

 the crura cerebri and meeting in the fore part of the fossa are the optic tracts, 

 decussating in the median plane to form the optic chiasm and continuing frontad 

 as the optic nerves. The arch of the optic tracts and optic chiasm and the crura 

 cerebri enclose the intercrural space, in which may be seen (1) the posterior per- 

 forated space (postperforatum; substantia perforata posterior); (2) the corpora 

 albicantia (c. mammillaria; c. candi cantia) ; (3) the tuber cinereum and the stalk of 

 the pituitary body. A groove marking the lateral boundary of the fossa along 

 each cms is termed the oculomotor sulcus, as the root fibres of the oculomotor 

 nerve have their superficial origin therein. The posterior perforated space is a 

 gray area with numerous minute apertures for the entrance of postperforant 

 branches of the posterior cerebral artery. The corpora albicantia are two small, 

 pea-like, white eminences closely set side by side. The tuber cinereum is a conical 

 projection between the corpora albicantia and the optic chiasm, to which the 

 hypophysis (pituitary body), resting in the sella of the sphenoid, is attached. In 

 the removal of the brain from the skull the stalk of the hypophysis is usually torn 

 through and an orifice, the lura, leading to the infundibular recess of the third 

 ventricle, is thus exposed. In the interval between the optic tract and the orbital 

 surface of each cerebral hemisphere is a small, depressed, triangular area of gray 

 substance leading laterad into the basisylvian fissure and dotted with numerous 

 apertures for the minute basal branches of the middle cerebral artery and called 

 the anterior perforated space (preperforatum). 



If the optic chiasm be drawn somewhat ventrad, a delicate gray lamina, the 

 lamina terminalis (terma; lamina cinera) is seen attached to the dorsal surface 



