INTERNAL STRUCTURE OF THE PONS 



829 



nuclei of the other motor cranial nerves, especially of the facial; (3) to the thalamus of the same 

 and chiefly the opposite side, and thus, through interpolation of thalamic neurones, their 

 impulses reach the somcesthetic area of the cerebral cortex. These fibres ascend in the recticular 

 formation of the opposite side, most of them finally coursing strictly within the medial lemniscus. 

 In crossing the mid-liae they contribute to the -internal arcuates. (4) Some fibres of both 

 the trigemruus direct and from its nucleus pass laterally into the cerebellum. The longer of 

 the reflex or association axones arising in the nucleus of termination may contribute to the 

 medial longitudinal fasciculus; many of them descend to terminate in the grey substance of the 

 spinal cord below the levels in which the ^bres of the spinal tract proper terminate. The 

 nucleus of termination is directly homologous to the nuclei of the fasciculus gracilis and fasci- 

 culus cuneatus, and, like the nuclei of termination of all sensory cranial nerves, it contains 

 cell-bodies homologous to those which give rise to the fascicuU proprii and commissural flbres 

 of the spinal cord. 



The masticator nerve [portio minor n. trigemini] is a purely motor nerve, usually called the 

 motor root of the trigeminus from the fact only that it makes its exit from the pons by the side 

 of the entering fibres of the trigeminus, passes outward over the ventro-mesial side of the 

 semilunar ganghon and accompanies the inferior maxiUary division (mandibular nerve) of the 

 trigeminus till it divides totally into its branches for the motor supply of the muscles of mastica- 

 tion. It serves, therefore, as but a relatively small part of the "motor root" of the trigeminus. 



The nucleus of origin of the masticator nerve is attenuated into two parts: (1) The chief 

 nucleus {nucleus princeps) lies on the dorso-medial side of the larger portion (sensory nucleus) 

 of the nucleus of termination of the trigeminus. It is the larger of the two parts and gives 

 origin to much the greater part of the masticator. (2) Scattered anteriorly and continuous 



Fig. 655. — Transverse Section Through Upper Part of Pons at the Level of the 

 Entrance of the Trigeminus. (From Villiger.) 



Anterior medullary velum 



Gowers' tract' 

 Fourth ventricle 



Brachium conjunctivum 



Mesencephalic root of 

 masticator nerve 



Fasc. long, dorsalis 

 (Schutz) 

 Medial longitudinal 

 fasciculus 



Nucleus reticu- 

 laris tegmenti 



Corpus trapez. 

 and medial 

 lemniscus 



Deep stratum of 

 pons 



Sensory nucleus of 



trigeminus 

 Chief nucleus of mas- 

 ticator nerve 

 Thalamo- 

 olivary tract 

 Lateral 

 lemniscus 



Brachium 

 pontis 



N. trigeminus 



Pyramidal f ascicuh 

 Superficial stratum of pons 



with the chief nucleus, in Ime with the locus coeruleus, are the cell-bodies usually described as 

 the nucleus of the mesencephalic (descending) root. These cells lie in decreasing linear distribu- 

 tion, through the mesencephalon, as far anterior as the posterior commissure of the cerebrum, 

 and the mesencephalic root of the nerve accumulates as it descends to join the exit of the 

 fibres arising from the chief nucleus. The average diameter of its cells is somewhat less than 

 for the chief nucleus. 



It is not clearly settled that the fibres arising from the mesencephahc nucleus of the masti- 

 cator nerve go to the muscles of mastication. As suggested by Kolliker, some of these may 

 supply the tensor veh palatini and tensor tympani muscles. Recent mvestigations of lower ani- 

 mals by Johnston and Willems indicate that the mesencephalic root may contain no motor 

 fibres at all, representing instead a portion of the sensory trigeminus fibres. It is claimed that 

 some fibres in descending give off collaterals which terminate about cells in the chief nucleus, 

 and thus an impulse descending by them is given a wider distribution and also reinforced by 

 the interpolation of another neurone. Such fibres, however, may be the sensory fibres just 

 mentioned terminating upon the cells of the nucleus to form simple reflex arcs. 



It is claimed that each masticator nerve receives a few fibres arising from the cells of the 

 nucleus of that of the opposite side. 



Both parts of the nucleus of the masticator receive afferent impulses brought in by the 

 trigeminus of the same (chiefly) and of the opposite side, and both receive cortical impulses 

 by fibres from the inferior portion of the precentral gyrus which descend in the cerebral ped- 

 uncles and cross to terminate in the nucleus of the opposite side. 



The internal structure of the pons. — The nuclei and roots of the trigeminus, masticator, 

 abducens, facial, glosso-palatine, cochlear and vestibular nerves are extended within the level 



