362 SPECIAL ANATOMY. 



Weight : from three and a half to four pounds and one-third ; 

 greatest longitudinal diameter, six inches ; transverse, five inches ; 

 vertical, four and three-quarter inches. It is divided into the three 

 following portions : 



615. 1. The great brain, cerebrum, fills up the cranial cavity, 

 with the exception of thefossce occipitales inferiores, is semi-oval, 

 four inches deep, about three pounds weight, and rests with its basis 

 upon the anterior and central cranial fossee, and upon the tentorium 

 cerebelli. A deep fissure divides the cerebrum into two lateral 

 halves or hemispheres, of equal size, each of which contains, in 

 the interior, a cavity (ventriculus lateralis), consists of three lobes, 

 and which, on the surface, are distinguished by the convolutions, 

 gyri, like intestines, which are limited by irregular fossae (suki) 

 not very deep, and which do not generally correspond upon the 

 two hemispheres. On the inferior surface of the brain we see, in 

 the middle from before to behind, proceeding from the centre of the 

 brain, also before and between the crura cerebri, a perforated place, 

 the substantia perforata media, before it two roundish tubercles, 

 corpora mamillaria y before these a gray enlargement, tuber cine- 

 reum, elongated into the infundibulum to which the glandula pi- 

 tuitaria is connected. Before the tuber cinereum the decussation 

 of the optic nerves presents itself. Upon both sides of the basis 

 cerebri we see the fossa Sylvii, which separates the anterior from 

 the middle cerebral lobes, and the origin of the twelve pairs of 

 nerves, of which the olfactory nerve is situated in a particular ex- 

 cavation. Behind that part of the hemispheres which is situated 

 over the cerebellum is a little hollowed out. On the external sur- 

 face of the brain we see a fissure, the continuation of the fossa 

 Sylvii, which separates it into a horizontal and ascending portion. 

 The superior surface presents the great longitudinal fissure, fissura 

 cerebri longitudinalis, and the irregular gyri and sulci. On the 

 internal surfaces of the hemispheres, which are turned towards 

 each other, there is a tolerably constant gyrus fornicatus which 

 forms an arch around the great cerebral commissure. We gene- 

 rally admit three lobes on each hemisphere ; but according to 

 Arnold and others we must receive five, an anterior, middle, supe- 

 rior, posterior, and the Insula or lobus intermedius (according to 

 Burdach, lobus caudicis). The last presents the gyri breves. 



616. 2. The small brain, cerebellum, is situated in the inferior 

 cranial fossa, covered by the tentorium. It projects a little behind 

 the posterior cerebral lobes, is roundish and flattened from above 

 downwards ; its transverse diameter measures four inches, its Jon- 



