'////; CEREBELLUM 787 



greatest relative size, and possesses a mediam or basilar sulcus in which lies the 

 basihir arici-y. Its sagittal dimension varies from 25 to 30 mm., while its trans- 

 verse dimension (longitudinal wit h the course of its fibres) is somewhat greater. It is 

 a rounded white prominence interposed between the cerebral peduncles (crura) above 

 and the medulla nblnngata below. Its injirior margin is rounded to form the in- 

 ferior pontine sulcus, which, between the points of the emergence of the pyramids, 

 is continuous with and transverse to the foramen ca'cuni. Its superior margin is 

 thicker and is rounded to form the superior pontine sulcus, which, between the 

 cerebral peduncles, is continuous with and transverse to the interpeduncular fossa. 

 It is bilaterally symmetrical. The ventro-lateral bulgings of its sides (and, therefore, 

 the basilar sulcus) are produced by the passage through it of the fibres of the cerebral 

 peduncles from above, to reappear as the pyramids below. Its ventral surface rests 

 upon the basilar process of the occipital bone and the dorsum sellse of the sphenoid, 

 while its lateral surfaces are adjacent to the posterior parts of the petrous portions 

 of the temporal bo'nes. 



The fibres of the thicker superior portion of the brachium pontis course obliquely 

 downwards to their entrance into the cerebellar hemisphere; those of the lower and 

 mid-portions course more transversely, naturally converging upon approaching 

 the cerebellum. Certain fibres of the upper mid-portion course at first transversely 

 and then turn abruptly downwards across the fibres above them, to join the inferior 

 portion of the brachium pontis. This bundle is termed the oblique fasciculus 

 (fig. 580). The trigeminus or fifth cranial nerve penetrates the superior lateral por- 

 tion of each brachium pontis near the point of the downward turn of the oblique 

 fasciculus; its large afferent and its small efferent roots accompany each other quite 

 closely. 



That portion of the rhombencephalon overlying the pons and forming the floor 

 of the fourth ventricle is not a part of the pons at all. It is merely a continuation 

 of the brain-stem from the medulla below to the structures above. Therefore on the 

 dorsal surface there is no line of demarcation between the pons and medulla below 

 or between the pons and isthmus above. The fibres of the trigeminus pass through 

 the pontine fibres to and from their nuclei in the brain-stem. 



THE CEREBELLUM 



The cerebellum or hind brain is the largest portion of the rhombencephalon. It 

 lies in the posterior or cerebellar fossa of the cranium, and behind the pons and 

 medulla oblongata, overhanging the latter. It fits under the occipital lobes of the 

 cerebral hemispheres, from which it is separated by a strong duplication of the inner 

 layer of the dura mater, the tentorium cerebelli. Its greatest diameter lies trans- 

 versely, and its average weight, exclusive of the dura mater, is about 140 gm., or 

 about 10 per cent, of the entire encephalon. It varies in development with the 

 cerebrum, and, like it, averages larger in the male. It is relatively larger in adults 

 than in children. Its development begins as a thickening of the anterior portion of 

 the roof (dorsal zone) of the posterior of the three primary brain vesicles. Resting 

 upon the. brain-stem, it roofs over the fourth ventricle and is connected with the 

 structures anterior, below, and posterior to it by its three pairs of peduncles. 



The surface of the cerebellum is thrown into numerous narrow folia or laminae, 

 which in the given localities run more or less parallel with each other. They are 

 separated by narrow but relatively deep sulci. Unlike the spinal cord and medulla, 

 in which the grey substance is centrally placed and surrounded by a mantle of white 

 substance, the surface of the cerebellum is itself a cortex of grey substance, the 

 cortical suhxtiinrc. enclosing a core of white substance, the mtdullary body. How- 

 ever, within this central core of white substance are situated definite grey masses, 

 the nuclei of the cerebellum. 



The gross divisions of the cerebellum are three: the two larger lateral portions, 

 the hemispheres, and between these the smaller central portion, the vermis. The 

 demarcation between these gross divisions is not very evident from the dorsal sur- 

 face, because the hemispheres in their extraordinary development in man encroach 

 upon the vermis, and, being pressed under the overlapping cerebral hemispheres, 

 they become partially fused upon the vermis along the dorsal mid-line. Though 

 differentiated simultaneously with the cerebellar hemispheres in the human foetus, 



