4:04 NERVOUS SYSTEM. 



greatest part of this portion is composed of a layer of gray 

 substance. 



The two lateral halves of the posterior portion of the me- 

 dulla are connected together by fibres arising from the gray 

 matter of the lateral tracts, or intermediary fasciculi, passing 

 obliquely, in a curved direction from behind forward, to the 

 raphe in the median line. There are also fibres passing from 

 before backward, to form a posterior commissure, and fibres 

 arising from the cells of the olivary bodies, which connect 

 the gray substance of the lateral halves. Commissural fibres 

 also connect the gray matter of the lateral tracts with the 

 corpora dentata of the olivary bodies, and the olivary bodies 

 with the cerebellum, their fibres forming part of the inferior 

 peduncle of the cerebellum. In addition, it is probable that 

 .fibres, taking their origin from all of the gray nodules of 

 the medulla, pass to the parts of the encephalon situated 

 above. 



As far as the fibres of origin of the nerves are concerned, 

 it may be stated in general terms that a number of the motor 

 roots arise from the gray matter of the floor of the fourth 

 ventricle, the roots of the sensory nerves arising from gray 

 matter in the posterior portions. 



Aside from purely anatomical demonstrations, the con- 

 nection of the anterior pyramids of the medulla with the cor- 

 pora striata has been shown by pathological observations. 

 It is well known that, when the connection between the 

 nerve-centres and the fibres is destroyed, these fibres after a 

 time become degenerated. In old lesions of the corpora 

 striata, Cruveilhier, Tiirk, and, more lately, Yulpian, have 

 shown that, when the white substance is injured upon one 

 side, there follow degeneration and atrophy of the fibres of 

 the corresponding cerebral peduncle and anterior pyramid 

 of the medulla, and of the lateral portion of the spinal cord 

 upon the opposite side. 1 This important fact illustrates the 

 connection between the lateral columns of the cord and the 



1 VULPIAN, Systeme nervevix, Paris, 1866, p. 470. 



