NORMAL HISTOLOGY. 



close proximity to the corpus dentatum : these are the dorsal or 

 outer and the mesial or inner accessory olivary nuclei, the 

 first of which lies behind the olivary nucleus, near and parallel to 

 its wavy band, while the second lies almost across the open end of 

 the corpus dentatum. 



Attention has already been directed to the tract of large nerve- 

 cells which lies near the median line and represents the nucleus 

 of the hypoglossal nerve. In the lower part of the medulla, 

 before the central canal opens out into the ventricle, a group of 

 numerous smaller cells lies close but dorsally to the nucleus just 

 mentioned ; as the central canal approaches the surface the tissues 

 forming its former dorsal border become gradually laterally displaced, 

 in consequence of which this group of nerve-cells then comes to lie 

 outside of the hypoglossal nucleus. These cells form a continuous 

 column throughout almost the length of the medulla, constituting 

 a common nucleus of the spinal accessory, pneumogastric, and 

 glosso-pharyngeal nerves. 



The four principal tracts of the medulla are made up chiefly 

 of the continuations of the columns of the cord ; without entering 

 into a detailed account of these structures, a brief outline of the 

 most important of the constituents of the tracts may here find 

 place. 



1. The anterior pyramid is composed of two sets of fibres : the 

 continuation of the direct pyramidal tract of the anterior column of 

 the cord, which does not take part in the decussation of the pyra- 

 mids, and the continuation of the crossed pyramidal tract of the 

 lateral column. After its formation in this manner the anterior 

 pyramid becomes divided into three very unequal groups : (a) the 

 majority of its fibres continue directly into and through the pons 

 to the cerebrum ; (b) certain fibres pass beneath the olive, join- 

 ing fibres from the latter to aid in forming the fillet; (c} a few 

 fibres are deflected to the restiform body and pass probably to the 

 cerebellum. 



2. The lateral tract claims all the fibres of the lateral column not 

 included in the crossed pyramidal and the direct cerebellar tract, 

 together with the external anterior or ground-bundle, since the latter 

 really is a part of the adjacent tract of the lateral column. The 

 antero-lateral fibres enter beneath and at the side of the anterior 

 pyramid and pass under the olivary body and the arcuate fibres to 

 take part in making up the formatio reticularis ; as the fasciculus teres 

 they appear in the floor of the fourth ventricle, and later in the teg- 

 mentum of the crus. 



3. The restiform body contains constituents from a number of 

 sources ; these may be arranged in two groups, those derived from 



