CHAP, ix.] NERVOUS SYSTEM AND ORGANS OF SENSE. 263 



which is an antero-posteriorly extending portion of white substance 

 (going to the Pons), called the anterior pyramid. Outside each such 

 pyramid is a small, more or less hidden, oval structure, termed the 

 olivary body, and external to and behind each of these is the Land 

 of nervous tissue already spoken of as the restiform body. The 

 middle part of the posterior surface of the medulla is occupied by the 

 posterior pyramids, which are placed one on each side of its posterior 

 median fissure, and which are continuous behind with the median 

 dorsal parts of the spinal cord, and in front seem to blend with the 

 restiform bodies. 



In order to see the inferior, or ventral, surface of the brain, it 

 must be removed from the cranial cavity, the cerebro-spinal axis 

 being cut through at the foramen magnum, i.e., at the hinder end of 

 the medulla oblongata. This being done, the cut surface of the 

 medulla will exhibit a doubly crescentic arrangement of internally 

 placed grey tissue, similar to that shown by the cut surface of the 

 spinal cord. If, however, sections of the medulla be made at points 

 further and further forwards, it will be seen that the grey matter 

 gradually becomes concentrated (as the medulla advances forwards) 

 near the middle of its dorsal surface. 



The ventral surface of the brain being in view, the two lateral 

 parts of the cerebellum are visible, one on each side of the medulla a 

 small process of each half called the floculus is connected by the 

 dura mater with that depression on the inner surface of the petrous 

 portion of the temporal bone, which was described as the cerebellar 

 fossa.* Continuing on we find on each side of the anterior end of 

 the anterior pyramids a conspicuous band of transverse fibres, each 

 of which is called a corpus trapezoidemn. Immediately in front of 

 these bands, is the transverse, convex eminence of the pons Varolii, 

 against the hinder margin of which the front ends of the anterior 

 pyramids abut. The corpora trapezoidea form a transverse band 

 which is interrupted by these pyramids, while the greater band of the 

 pons Varolii is uninterrupted by them. The pons lies upon the anterior 

 part of the basi-occipital bone, and is medianly grooved by an antero- 

 posterior shallow depression, along which runs the basilar artery. 



Emerging from the front of the pons are two masses of white sub- 

 stances marked with longitudinal stria3, and made up of longitudinal 

 fibres (the crura ccrebri) which diverge as they advance, and are 

 crossed superficially by two anteriorly converging round cords, the 

 optic tracts (which unite to form the optic nerves), and ^ thus a 

 lozenge-shaped space is enclosed. At their opposite extremities the 

 optic tracts run upwards and backwards to the corpora geniculata 

 already noticed. In the hinder part of this space (called inter- 

 peduncular) is a small rounded mass, the corpus albicans, which shows 

 an indication of a median division into two lateral halves termed 

 corpora mammillaria. In front of this is a slight prominence termed 

 the tuber cincrcum, from the middle of which projects a hollow 



* See ante, p. CG. 



