THE NERVOUS SYSTEM. 



373 



mids, passing between the first and second transverse layers of fibres of 

 the pons, are continued into the base of the cerebral peduncles. The 

 posterior columns of the cord, lastly, chiefly constitute the fasciculi gra- 

 ciles and cuneati, the latter of which, in great part, enter the pedunculi 

 cerebelli, whilst the remainder, and the fasciculi graciles, situated exter- 

 nally to the eminentice teretes, may be traced into the tegmentum of the 

 crura cerebri. All these fasciculi consist, besides the gray substance, 

 of parallel nerve-fibres of the same dimensions as those of the cord, that 

 is to say, from 0'001-0'004 of a line, seldom more. 



Besides this white substance, the pons Varolii and medulla oblongata, 

 omitting the roots of the nerves, present a system of mostly horizontal 



fibres. This system consists : 1, of the well-known transverse, arcuate 

 fibres, external to the corpora pyramidalia and olivaria ; 2, of straight 



FIG. 146. Transverse section through the medulla oblongata in Man. P, pyramids; O, 

 olivary bodies ; F.I, fasciculus lateralis; F.c, fasciculi cuneati ; F.g,fasc.graciles; H, root of 

 hypoglossal nerve; F, root of n. vagus ; F.a,fissura anterior; F.p,fissura posterior in the floor 

 of the fourth ventricle, or rhomboid fossa; R, raphe : "a, longitudinal fibres of the raphe ; 6, 

 central gray layer with transverse fibres ; c, expansion of these fibres in the olivary column 

 and body ; d, accessory olivary nucleus ; e, hypoglossal nucleus /, decussation of the hypo- 

 glossal nerve ; g, nucleus of the vagus ; hhh, larger nerve-cells in the restiform bodies ; t, 

 medullary mass in the interior of the olivary body, belonging to the internal transverse 

 fibres; &, arcuate fibres external to the olivary body ; I, transverse fibres external to the 

 pyramids; m, , o, gray nuclei in the pyramids and olivary columns. Magnified 15 

 diameters. 



