THE CENTRAL NERVOUS SYSTEM. 



297 



becomes laterally displaced, while the remains of the base of 

 the anterior horn come to the surface of the ventricular floor, 

 and assist in forming the projection of the funiculus teres. A 

 longitudinal column of large nerve-cells occupies part of this, forming 

 the nucleus from which the numerous bundles of the roots of the 

 hypoglossal nerve arise. 



The changes affecting the anterior cornuaof the cord are produced 

 primarily by the decussation of those fibres of the lateral column 



FIG. 331. 



Diagram of lower end of medulla at level of 

 decussation of anterior pyramids : a, anterior 

 pyramidal tracts ; b, posterior median septum ; 

 c, fibres of crossed pyramidal tracts crossing 

 (d) to anterior pyramid of opposite side ; e, 

 anterior horn of gray matter isolated by decus- 

 sating fibres ; f, remains of bases of anterior 

 horns ; g, nucleus gracilis ; h, enlarged and 

 displaced posterior horns of gray matter. 

 (After Testut-Duval.) 



Diagram of medulla through lower part of olivary 



body : a, anterior pyramidal tract ; 3, posterior 

 median groove ; c, gray matter representing bases 

 of anterior cornua, the latter lying isolated at e, 

 forming nucleus lateralis ; d, decussating fibres of 

 formatio reticularis ; g, nucleus gracilis ; h, gray 

 matter of bases of posterior horns ; k, nucleus 

 cuneatus ; t, remains of posterior horns, substantia 

 gelatinosa of Rolando ; /, ascending root of tri- 

 facial nerve ; m, pneumogastric, n, hypoglossal, 

 nerve ; o, nucleus dentatus of olive ; /, mesial 

 accessory olive ; s, sensory portion of anterior 

 pyramids. (After Testut-Duval.) 



which contribute to the formation of the anterior pyramids. The 

 fibres of the crossed pyramidal tract, in taking the shortest course 

 to reach the point of decussation, cut obliquely through the gray 

 substance in such a manner that the anterior cornu becomes broken 

 up, its caput being entirely separated ; the remaining portion of 

 its base forms a small mass of gray matter lying ventro-laterally to 

 the central canal. The isolated segment of the anterior cornu is 

 pushed to the side by the development of the pyramid, and, higher 

 up, by the additional displacement caused by the appearance of the 

 olivary body between the caput cornu and the pyramid ; in conse- 

 quence the separated part is displaced both laterally and dor- 

 sally, and becomes the lateral nucleus, taking up a position in 

 close relation with the now ventrally situated posterior horn. By 

 the penetration of transverse and longitudinal fibres the greater part 



