38 THE MEDULLA OBLONGATA AND PONS VAROLII. 



THE BRAIN OB ENCEPHALON. 



The brain or encephalon comprises the medulla oblongata and pons Varolii, the 

 cerebellum, the mid-brain, and the cerebrum. The medulla oblongata (fig. 28, D) is 

 the part continuous with the spinal cord ; it is the lowest part of the encephalon, 

 and rests against the basilar process of the occipital bone. The pons Varolii (c) is 

 a direct continuation upwards and forwards of the medulla oblongata, but the name 

 also includes the transversely coursing fibres of the middle cerebellar peduncle. The 

 cerebellum (B) occupies the posterior fossa of the cranium. Its central part forms the 

 dorsal boundary of a space, which is bounded ventrally by the dorsal surface of the 

 medulla oblongata and pons Yarolii, and which is named the fourth ventricle of the 



Fig. 28. PLAN IN OUTLINE OP THE EN- 

 CEPHALON, AS SEEN FROM THE KIQHT 

 SIDE. 1 



The parts are represented as separated 

 from one another somewhat more than 

 natural so as to show their connections. 

 A, cerebrum ; e. fissure of Sylvius ; B, 

 cerebellum ; C, poiis Varolii ; D, medulla 

 oblongata ; a, peduncles of the cerebrum ; 

 b, c, d, superior, middle, and inferior 

 peduncles of the cerebellum ; the parts 

 marked a, I, form the isthmus ence- 

 phali. 



brain. The cerebellum is con- 

 nected by peduncles below with 

 the medulla oblongata, above with 

 the cerebrum, and in the middle 

 with the pons Varolii (c). The 

 mid-brain is continued from the 

 pons Varolii and comprises the two crura of the cerebrum (a), which are surmounted 

 in man and mammals by two pairs of tubercles named the corpora quadrigemina (&), 

 but in oviparous vertebrates by one pair only, then named the corpora bigemina or optic 

 lobes, The cerebrum (A) includes all the remaining and by far the largest part of 

 the encephalon. It is united with the parts below by the mid-brain, which forms a 

 comparatively narrow and constricted portion or isthmus. The cerebrum is mainly 

 composed of the large convoluted cerebral hemispheres, and within it are the third 

 and two lateral ventricles. It occupies the vault of the cranium, the anterior and 

 middle cranial fossae, and the superior fossae of the occipital bone. The cerebral 

 hemispheres are united together by a large commissure termed the corpus callosum, 

 and by smaller commissures (anterior, middle, and posterior). 



THE MEDULLA OBLONGATA AND PONS VAROLII. 



EXTERNAL CHARACTERS. 



The medulla oblongata or spinal bulb (bulbus rhachiticus, Meckel) is continuous 

 below with the spinal cord, on a level with the lower margin of the foramen magnum. 

 Its upper limit is marked off on the ventral aspect from the pons Varolii, into which it 

 is continued above, by the lower margin of the mass of transverse fibres which unite 

 the two hemispheres of the cerebellum ; its ventral surface rests in the basilar groove 



