THE STRUCTURE OF THE MEDULLA OBLONGATA 



139 



fibers of the fasciculus longitudinalis medialis and the tectospinal tract (Fig. 

 101). The decussation of the lemniscus begins at the upper border of the 

 decussation of the pyramids, where the sensory fibers are grouped into coarse 

 bundles arching around the central gray matter (Fig. 99), and extends as far as 

 do the gracile and cuneate nuclei, that is, to about the middle of the olive. In 

 sections through the lower half of the olive the internal arcuate fibers describe 

 broad curves through the reticular formation and their decussation occupies a 

 considerable ventrodorsal extent of the raphe (Fig. 101). 



Nerve cell in the nucleus cuneatus 



Ramification of fibers from the fasciculus cuneatus 



Nucleus cuneatus 



Fasciculus 

 cuneatus 



Subslantia 

 gelatinosa 



Fig. 100. From a transverse section through the medulla oblongata of a kitten, to illustrate 

 the termination of the fibers of the fasciculus cuneatus, and at a the beginning of the internal 

 arcuate fibers. (Combined from drawings by Cajal.) 



The arcuate fibers of the medulla oblongata may be separated into two 

 groups: those which run through the reticular formation constitute the inter- 

 nal arcuate fibers; and those which run over the surface of the medulla, the 

 external arcuate fibers. The internal arcuate fibers are of at least three kinds: 

 (1) those described in the preceding paragraph, which arise in the gracile and 

 cuneate nuclei and form the medial lemniscus; (2) sensory fibers of the second 

 order, arising in the sensory nuclei of the cranial nerves; and (3) olivocerebellar. 

 fibers, which will be considered in another paragraph. Our knowledge of the 

 external arcuate fibers is less satisfactory. From the nuclei of the posterior funic- 



