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 receptive nucleus of the 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. The proportion of the crossed to the uncrossed 

 fibres varies greatly ; while usually from three to ten per cent, of 

 the pyramidal fibres pass directly into the anterior columns of the 

 spinal cord, total decussation of these fibres takes place in about 

 eleven per cent., in such cases the anterior pyramidal tract being 

 evidently wanting. In only about sixty per cent, is a symmetrical 

 disposition of the two pyramidal tracts on each side observed. 



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 ; the sensory fibres 

 derived from the posterior columns after crossing in the sensory 

 decussation pass brainward and aid in forming the mesial fillet. 



3. The restiform body contains constituents from a number of 

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



