804 THE NERVOUS SYSTEM 



2. THE PONS 



The pons (Varoli) is, for the most part, a great commissure or 'bridge' of 

 white substance coursing about the ventral aspect of the brain-stem, and connect- 

 ing the cerebellar hemisphere of one side with that of the other. In addition it 

 contains fibres passing both to and from the structures of the brain-stem and the 

 grey substance of the cerebellum, and fibres descending from the cerebral cortex. 

 Each of its lateral halves forms the middle of the three cerebellar peduncles, the 

 brachium pontis of either side. 



In size it naturally varies directly with the development of the cerebellum, both 

 in a given animal and relatively throughout the animal series. In man it attains 

 its greatest relative size, and possesses a median or basilar sulcus in which lies 

 the basilar artery. Its sagittal dimension varies from 25 to 30 mm., while its 

 transverse dimension (longitudinal with the course of its fibres) is somewhat 

 greater. It is a rounded white prominence interposed between the visible portion 

 of the cerebral peduncles (crura) above and the medulla oblongata below. Its 

 inferior 7??argrm is rounded to form the inferior pontine sulcus, which, between the 

 points of the emergence of the pyramids, is continuous with and transverse to 

 the foramen caecum. 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. (See fig. 629.) It is bilaterally sym- 

 metrical. 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 pedun- 

 cles 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 bones. 



The fibres of the thicker superior portion of the pons {fasciculus superior pontis) course 

 obliquely downward to their entrance into the brachium of the pons and the cerebellar hemis- 

 phere; those of the lower and mid-portions {fasciculus medius pontis) course more transversely, 

 naturally converging upon approaching the cerebellum. Certain fibres of the upper mid- 

 portion course at first transversely and then turn abruptly downward across the fibres above 

 them, to join the inferior portion of the brachium pontis. This bundle is termed the oblique 

 fasciculus (fig. 629). 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 root and the masticator nerve (its small efferent root) accompany each other quite 

 closely. On either side of the basal surface of the pons usually may be seen a small bundle 

 of fibres which begins in the interpeduncular fossa, near or in the sulcus of the oculomotor 

 nerve. It passes laterally along or under the superior border of the pons, loses some of its fibres 

 in the lateral sulcus of the mesencephalon, then runs inferiorly between the superior cerebellar 

 peduncle and the brachium of the pons to disappear in the junction of these. Being sometimes 

 double, it is known as the lateral filaments of the pons {fda lateralia pontis or tania pontis). 

 The location of the cell-bodies giving origin to it is uncertain. 



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

 fourth ventricle is not really 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 fine of demarcation between the pons and medulla below or between the pons and isthmus 

 above. The fibres of the trigeminus and masticator nerve pass through the pontine fibres to 

 and from their nuclei in the brain-stem. 



3. 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 dorsal to 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 ientoriurn cerebelli. 



Its greatest diameter lies transversely, 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 tlic 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 po.stcrior of the three primary brain vesicles. Resting upon the brain- 

 stem, it roofs over the fourth ventricle and is connected with tlie structures anterior, below, 

 and posterior to it by its tiirce pairs of jjeduncles. 



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

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



