ELEMENTS OF MAMMALIAN ANATOMY. 



passes through the foramen magnum. The pons Varolii 

 is the bridge of transverse fibers seen on the ventral 

 aspect of the brain just cephalad of the medulla (Fig. 94). 

 The cerebellum or little brain lies on the dorsal side of 

 the medulla and is partly covered by the caudal part of 

 the cerebrum. The mesencephalon is not visible exter- 

 nally on the dorsal aspect, but may be seen immediately 

 cephalad of the cerebellum by cutting away the cerebrum 

 (Fig. 98). The diencephalon is visible externally only 

 on the ventral aspect of the uncut brain, where it forms 

 the floor of the third ventricle. It may be viewed as a 

 whole if the dorsal half of the cerebrum including the 

 corpus callosum is cut away. The telencephalon is com- 

 posed of the two large hemispheres partially surrounding 

 the diencephalon and mesencephalon. 



The brain is composed of two kinds of matter, white 

 and gray. The former is constructed for the most part 

 of fibers, while the latter is formed largely of cells. The 

 outer or cortical portion of the cerebrum and cerebellum 

 is a layer of gray matter less than a half centimeter 

 thick, and is disposed in folds called gyri or convolutions 

 with intervening depressions termed sulci, the more im- 

 portant of which are called fissures. 



The four different portions of the cerebrum are known 

 as frontal, parietal, occipital, and temporal lobes, which 

 occupy the respective regions of the cranial cavity. The 

 frontal and parietal lobes are separated by the crucial 

 fissure, extending transversely between them. The ol- 

 factory lobe (usually torn off in removing the brain 

 from the skull) projects from the cephalic portion of the 

 frontal lobe. The parietal lobe is marked by three gyri, 

 named according to location gyrus marginalis, gyrus 

 suprasylvius, and gyrus ectosylvius (Figs. 89 and 90). 

 The caudal portions of the gyri marginalis and supra- 



