INTERNAL STRUCTURE OF PONS 545 



there is a large amount of grey matter which forms the nuclei pontis. 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 encroaching 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 acustic 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 lateral parts 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 pons a 

 dark spot in the lateral part of the floor indicates the position of a small 

 mass of pigmented cells called the substantia ferruginea. It underlies 

 the locus coeruleus. 



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 the brachium conjunctivum to gain the 

 surface. 



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

 down 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 corpus 

 trapezoidum. 



VOL. II 35 



