THE MEDULLA OBLONGATA. 693 



short dendrites, which spread out in a spider-like manner in the granular layer. 

 Their axons pass outward into the molecular layer, and, bifurcating at right angles, 

 run horizontally for some distance. In the outer part of the granular layer are 

 also to be observed some larger cells, of the type termed Crolgi cells (Fig. 367). 

 Their axons undergo frequent division as soon as they leave the nerve-cells, and 

 pass into the granular layer, while their dendrites ramify chiefly in the molecular 

 layer. 



Finally, in the gray matter of the cerebellar cortex fibres are to be seen which 

 come from the white centre and penetrate the cortex. The cell origin of these 

 fibres is unknown, though it is believed that it is probably in the gray matter of 

 the spinal cord. Some of these fibres end in the granular layer, by dividing into 

 numerous branches, on which are to be seen peculiar moss-like appendages ; hence 

 they have been termed by Ramon y Cajal the " moss fibres" ; they form an arbor- 

 escence around the cells of the granular layer. Other fibres derived from the 

 medullary centre can be traced into the molecular layer, where their branches 

 cling around the dendrites of Purkinje's cells, and hence they have been named 

 the clinging or tendril fibres. 



2. The independent centres of gray matter in the cerebellum are four in number 

 on each side : one is of large size, and is known as the corpus dentatum ; the other 

 three, much smaller, are situated near the middle of the cerebellum, and are known 

 as the nucleus emboliformis, nucleus globosus, and nucleus fastigii. 



The corpus dentatum or ganglion of the cerebellum is situated a little to the 

 inner side of the centre of the stem of the white matter of the hemisphere. It 

 consists of an irregularly folded lamina of a grayish-yellow color, containing white 

 fibres, and presenting on its antero-internal aspect an opening, the hilum, from 

 which most of the fibres of the superior cerebellar peduncle emerge. 



The nucleus emboliformis is a mass of gray matter placed immediately to the 

 inner side of the corpus dentatum, and partly covering its hilum. The nucleus 

 globosus is an elongated mass of gray matter, directed antero-posteriorly, and 

 placed to the inner side of the preceding. The nucleus fastigii is sbmewhat larger 

 than the other two, and is situated close to the middle line at the anterior end of 

 the superior vermiform process, and immediately over the roof of the fourth ven- 

 tricle, from which it is separated by a thin layer of white matter. It is known as 

 the roof nucleus of Stilling. 



Weight of the Cerebellum. Its average weight in the male is about 5 oz., 4 drs. 

 It attains its maximum weight between the twenty-fifth and fortieth years, its 

 increase in weight after the fourteenth year being relatively greater in the female 

 than in the male. The proportion between the cerebellum and cerebrum is, in the 

 male, as 1 to 8^-, and in the female as 1 to 8. In the infant the cerebellum is 

 proportionately much smaller than in the adult, the relation between it and the cere- 

 brum being, according to Chaussier, between 1 to 13, and 1 to 26 ; by Cruveilhier 

 the proportion was found to be 1 to 20. 



V. The Medulla Oblongata (Fig. 370). 



The medulla oblongata or metencephalon, known also as the spinal bulb, is 

 the lowest division of the encephalon, and is continuous with the spinal cord. 

 It is developed from the fifth cerebral vesicle, the cavity of which forms thejower 

 half of the fourth ventricle. It extends from the lower margin of the pons Varolii 

 to a plane passing transversely just below the decussation of the pyramids, at which 

 level the spinal cord commences. This plane corresponds to the lower margin of 

 the foramen magnum. The upper limit of the medulla is marked off from the pons 

 Varolii on its ventral aspect by the abrupt lower margin of the latter. 



The medulla oblongata is directed from above obliquely dowmyard and back- 

 ward ; its ventral surface rests on the basilar groove of the occipital bone, while 

 its dorsal surface is received into the fossa between the hemispheres of the cere- 

 bellum, and forms the lower part of the floor of the fourth ventricle. It is pyramidal 



