THE HIND-BRAIN. 683 



The tegmental or dorsal portion of the pons is chiefly constituted by a con- 

 tinuation upward of the reticular formation and gray matter of the medulla. It 

 is subdivided into lateral halves by a median raphe continuous with that of the 

 medulla, but this does not extend into the ventral half of the pons, being here 

 obliterated by the transverse fibres. 



The dorsal portion of the pons, like the ventral, contains both transverse and 

 longitudinal fibres. The transverse fibres are collected into a distinct bundle, 

 which, from its shape, is sometimes termed the trapezium or corpus trapezoides. 

 It consists of fibres which proceed laterally to become connected with the cells of 

 the accessory auditory nucleus. The longitudinal fibres, which are continuous 

 Avith those of the medulla, are mostly collected into two bundles on either side. 

 One of these lies between the corpus trapezoides and the formatio reticularis of 

 the pons, and is a continuation upward of the sensory tracts ; it is termed the 

 fillet. The other bundle is situated more dorsally, near the floor of the fourth 

 ventricle ; it is the posterior longitudinal bundle, and contains both ascending and 

 descending fibres. Other longitudinal fibres, which are more diffusely distributed, 

 arise from the cells of the gray matter of the pons itself. The greater part of the 

 dorsal portion of the pons is, as stated above, a continuation upward of the 

 forrnatio reticularis of the medulla, and, like it, presents, on transverse section, 

 viewed under a moderate magnifying power, a reticular appearance. In addition 

 to the gray matter, which presents a number of small reticularly arranged masses, 

 with nerve-cells, there are some important collections of nerve-cells which require 

 mention. 



1. The superior olivary nucleus is a small isolated collection of gray matter, 

 situated on the dorsal surface of the outer part of the trapezium. In structure it 

 resembles the inferior olivary nucleus of the medulla, presently to be described, 

 and is situated immediately above it. The nerve-fibres derived from its cells pass 

 into the trapezium, and, as stated above, cross the middle line and enter the acces- 

 sory auditory nucleus of the other side. The other collections of nerve-cells in the 

 formatio reticularis of the pons are nuclei from which some of the cranial nerves 

 arise. 



2. Nuclei of the Fifth Nerve. The nuclei of the fifth nerve in the pons are two 

 in number : one for the motor root and the other for the sensory. The motor 

 nucleus is situated in the higher portion of the pons, close under the dorsal surface 

 and along the line of the lateral margin of the fourth ventricle. The sensory 

 nucleus lies external to the motor one, beneath the superior peduncle of the cere- 

 bellum, which forms the lateral boundary of the upper half of the fourth ventricle. 

 Some of the fibres from these nuclei pass to the raphe 1 of the pons, and thence 

 probably to the higher parts of the brain ; the rest form the nerve-roots of the 

 motor and sensory parts of the fifth nerve respectively. They pass through the 

 pons to emerge on its ventral surface at its lateral and constricted portion, nearer 

 its superior than its inferior margin. It must be mentioned that the whole of the 

 roots of the fifth nerve are not formed from these nuclei. The sensory root is 

 partly formed by a long tract of fibres, known as the ascending root, which can 

 be traced through the pons and medulla to the upper part of the spinal cord. The 

 motor root, in like manner, is partly formed by a long tract of fibres, which passes 

 downward from the gray matter in the floor of the Sylvian aqueduct and which is 

 termed the descending root. 



3. The nucleus of the sixth nerve is situated beneath the floor of the fourth 

 ventricle, on either side of the middle line. It lies close to the root of the facial 

 nerve, immediately to be described, being a little external to and beneath it, and 

 corresponds to the upper half of the fasciculus teres of the floor of the fourth 

 ventricle (Fig. 371). The fibres pass through the substance of the pons, and 

 emerge at the lower margin of this structure, between it and the upper end of 

 the medulla. 



4. The nucleus of the facial nerve is of elongated form, and is situated deeply 

 in the reticular formation below the floor of the fourth ventricle and dorsal to 



