56 THE MEDULLA OBLONGATA AND PONS VAROLII. 



system of the trapezium (p. 47). Higher up this nucleus blends with the ganglion 

 of the lateral root, the two together forming a ventral nucleus for the auditory 

 nerve (fig. 49, VIII.v.}. 



Towards the upper part of the bulb an extensive tract of grey matter containing 

 small scattered nerve-cells becomes developed outside the vago-glossopharyngeal 

 nucleus. This tract corresponds to the lateral triangular area (trigonum acustici) 

 which is seen on the surface outside the ala cinerea. Into it most of the fibres 

 of the ventral or vestibular division of tbe auditory nerve apparently pass ; it is 

 termed the inner or dorsal auditory nucleus (fig. 47, n.VIIl.p). Ventral 

 to this nucleus is a collection of grey matter with large nerve-cells, the nucleus of 

 Deiters. Its cells appear to be connected with the cerebellum, for it becomes 

 atrophied after removal of the cerebellar hemisphere of the same side in the new- 

 born animal. 1 Associated with this group of cells is a longitudinal bundle of nerve- 

 fibres which has been termed by Roller the ascending root of the auditory nerve 

 (fig. 49, /?.), but the precise connection of these fibres with the roots of the eighth 

 nerve has yet to be established. Most of these collections of cells will be again 

 noticed in dealing with the structure of the pons. 



The nerve-cells in the hypoglossal nucleus are largest ; those in the principal 

 nucleus of the spinal accessory of moderate size, and those in the vago-glossopharyn- 

 geal nucleus are small and fusiform : those of the principal (dorsal) auditory nucleus 

 are the smallest. There are a number of small cells in the ventral part of the hypo- 

 glossal nucleus (small-celled hypoglossal nucleus of Roller), but they do not give 

 origin to any of the fibres of the nerve (Forel). 



Nucleus of the olivary body. Besides those collections of grey matter which 

 are traceable from the grey matter of the spinal cord, portions occur in certain parts 

 of the medulla oblongata, which are not represented in the cord. Of these the most 

 striking is the nucleus of the olivary ~body, which has been termed, from its appear- 

 ance in section, the corpus dentatum of the olive (fig. 44). It is enclosed in the 

 olivary prominence, and is therefore situated in the lateral area of the bulb, but 

 the grey matter is not visible from the surface, being covered by both longitudinal 

 and transverse white fibres. It takes the form of a thin wavy lamina, which is 

 curved round at its edge so as to form an ovoid scalloped capsule. The open part or 

 hilum of this looks towards the middle line and receives a considerable tract of white 

 fibres, which emanate from the raphe, being derived to all appearance from the 

 opposite olive, and pass into the hilum along its whole extent, forming the so- 

 called olivary peduncle (p.o.). 2 Under the microscope the nucleus appears as a wavy 

 band of neuroglia, with small multipolar nerve-cells embedded in it. The fibres of 

 the olivary peduncle diverge as they pass to the grey lamina. They are partly lost 

 in the grey matter of the olivary nucleus but mostly pass in small bundles through 

 the lamina, those which are more posterior turning backwards and coursing obliquely 

 through the posterior part of the lateral area to join the restiform body and thus to 

 pass to the cerebellum as internal arched fibres. These internal arched fibres are 

 easily distinguishable by their small diameter from the large internal arched fibres 

 which belong to the tract of the fillet : moreover they develope their myelin sheath 

 later. Others after coursing through the grey lamina and running between the longi- 

 tudinal fibres which cover the olive reach the surface, where they bend round and are 

 continued as part of the layer of external arched fibres into the restiform body. 

 Through the restiform body, the arched fibres and the fibres of the olivary peduncles, 

 the cerebellar hemisphere of one side is connected therefore with the olivary nuclei 



1 The nucleus of Deiters was formerly regarded as giving origin to part of the auditory nerve, and 

 is also known as the outer or superior auditory nucleus (see p. 62). 



2 Some fibres emerge from the hilum and turn sharply round the ventral and lateral borders of the 

 dentate nucleus, to which they form a kind of capsule (fig. 44, s.o. ). 



