THE BRAIN IN THE EDENTATA. 359 



I have already had occasion to refer to the pons Varolii in Orycteropus. It is a 

 prominent transvei-se strand (fig. 1) of nerve-fibres which crosses the ventral surface of 

 the brain-stem, and, after bending up on the lateral aspect of the brain-stem on each side, 

 dips into the cerebellum, forming its middle pedu.ncle. In the mesial plane the pons Varolii 

 is 10 mm. broad and 3 mm. thick. It expands slightly on each side of the mesial plane, 

 and then contracts again so that at a distance of 10 mm. from the mesial plane it is 

 merely 6 mm. broad. At this point it splits into two strands to enclose the large fifth 

 nerve, the great majority of the fibres passing in front of the nerve. At the lateral 

 margin of the nerve the fibres collect again to form a rounded cord, the middle peduncle 

 of the cerebellum, which bends round the lateral aspect of the brain-stem, and after a 

 course of 13 mm. in the dorsal direction disappears in the cerebellum. 



In Tamandua (fig. 9), Myrmecophaga, and the Bradi/podidce, the pons presents similar 

 features, but is relatively larger than it is in Orycteropus. In Manis and the Brady- 

 podidic its features are similar. In Chlamydophorus the pons is a very insignificant 

 bundle of fibres. 



On the ventral surface of the medulla oblongata a very prominent band of fibres makes 

 its appearance in Tamandua (fig. 9), by coming from under cover of the pons Varolii on 

 each side of the mesial plane. These are the pyramidal tracts or anterior pyramids. 

 They are equally weU-developed in 2Iyr}nccopltag(i, and only slightly less prominent in 

 the Sloths and Armadillos. But in Orcteropus (fig. 1) the pyramidal tracts are flattened 

 bands, the position of which is indicated by the emergence of the roots of the hypo- 

 glossal nerve along their lateral borders. The contrast between these insignificant bands 

 and the prominent cords in the Ant-eaters is probably explained by the larger pallium in 

 the latter, because the fibres of the pyramids are derived from it. 



Just behind the pons a transverse band of nerve-fibres, the corpus trapezoldemn, 

 emerges from under cover of the pyramid on each side and pursues a course to the lateral 

 border of the brain-stem. Just behiud the situation of the fifth nerve, another and much 

 smaller nerve — the seventh or facial — separates this trapezium (or trapezoid) bodv from 

 the pons Varolii. Upon tracing the trapezium a little beyond the seventh nerve it 

 leads to another nerve or pair of nerves, each of which is larger than the seventh. This is 

 the auditory or eighth nerve, composed of two parts, cochlear and vestibular respectivelv. 

 These nerves immediately dip into a large mass of grey substance, which we may dis- 

 tinguish as the tulierculum ac/istlc/im laterale (fig. 27). This lateral acoustic tubercle is 

 an elongated band of grey substance, of a somewhat crescentic shape, which is jslaced 

 mainly upon the lateral aspect of the brain-stem, at the junction of the medulla oblongata 

 and the pons. It begins ventrally just opposite the lateral margin of the emerging fifth 

 nerve, and is placed in the groove behind the middle peduncle of the cerebellum: it 

 proceeds obliquely upward and backward, and upon reaching the dorsal aspect of the 

 brain-stem it extends directly inward, closely embracing the middle peduncle of the 

 cerebeUuni in the whole of its course. The greater part of this tubercle lies within the 

 fourth ventricle, since the line of attachment of the epithelial roof of this cavity crosses 

 over the extreme lateral margin of the acoustic tubercle. 



Three large and compact bimdles of nerve-fibres enter the cerebellum on each side of 



49* 



