5 02 THE BRAIN 



wards from the bases of the pedunculi cerebri ; some of the longitudinal 

 fibres are cerebro-spinal fibres, passing to the medulla oblongata where 

 they form the bundles of the pyramids. Others are C( rebro-pontine fibres ; 

 they terminate round the cells of the nuclei pont is, which are the small 

 masses of grey matter which occupy the interstices between the transverse 

 and longitudinal bundles of fibres. Of the transverse fibres two distinct 

 sets may be recognised, viz., the superficial transverse fibres, through 

 the midst of which the bundles of cerebro-spinal fibres are prolonged, 

 and a deeper set termed the corpus trapezoidum. The superficial transverse 

 fibres traverse the entire thickness of the ventral part of the pons, and 

 on each side, pass into the corresponding brachium pontis. The trapezial 

 fibres lie posterior to the cerebro-spinal bundles in the boundary area 

 between the dorsal and ventral parts of the pons, but encroach con- 

 siderably into the ground of the former. They are seen only in the lower 

 part of the pons, and they pass into the lateral lemniscus. They take origin 

 in the terminal nucleus of the cochlear division of the acoustic nerve. 



The dorsal or tegmental part of the pons is, for the most part, formed 

 of a prolongation upwards of the formatio reticularis of the medulla. 

 Superiorly, it is carried into the tegmental parts of the pedunculi cerebri. 

 It is divided into two halves by a median raphe, which is continuous, 

 below, with the raphe of the medulla oblongata, and, above, with the raphe 

 of the tegmental part of the mesencephalon, whilst over its dorsal surface is 

 spread a thick layer of grey matter which belongs to the upper part of the 

 floor of the fourth ventricle. In transverse sections through the upper 

 part of the pons 'a dark spot in the lateral part of the floor indicates the 

 position of a small mass of pigmented cells called the substantia ferruginca. 

 It underlies the locus cceruleus. 



Four strands of longitudinal fibres are seen on each side in transverse 

 sections through the dorsal part of the pons. These are (i) the medial 

 lemniscus, (2) the lateral lemniscus, (3) the medial longitudinal bundle, and 

 (4) the brachium conjunctivum. 



The medial lemniscus assumes, in the pons, a ribbon-shaped form. It is 

 placed between the ventral part of the pons and the formatio reticularis 

 of the dorsal part. 



The lateral lemniscus, largely composed of fibres derived directly or in- 

 directly from the corpus trapezoidum, is seen in the upper part of the pons. 

 It sweeps round the lateral side of trie brachium conjunctivum to gain the 

 surface. 



The medial longitudinal bundle is much more distinct than it is in 

 the medulla oblongata. It has separated itself more completely from 

 the longitudinal fibres of the formatio reticularis, and it is now seen, 

 close to the median plane, immediately subjacent to the grey matter of 

 the floor of the fourth ventricle. 



The brachium conjunctivum, in transverse sections, presents a semi- 

 lunar outline. It occupies a lateral position in the dorsal part of the 

 pons, and gradually sinks deeply into its substance, although it does not 

 become completely submerged until it reaches the mesencephalon. 



The superior olive is a small isolated clump of grey matter which is 

 embedded in the dorsal part of the pons in the path of the fibres of the 

 corpus trapezoidum. 



CRANIAL TOPOGRAPHY 



After the gyri and sulci of the hemispheres and the various parts of the 

 mid-brain and the hind-brain have been carefully studied the dissectors 



