THE MEDULLA OBLONGATA. 503 



fibres make their appearance, passing from one side to the other. The 

 greater part of these fibres come from the continuations of the lateral 

 and posterior columns, and from the posterior horns of gray matter, 

 whence they pass forward and inward, and cross each other on the 

 median line at the place previously occupied by the white commissure. 

 The increasing number of these interchanging fibres, and the fact that 

 they cross the median line in bundles of considerable size, and in an ob- 

 lique direction from below upward, produce the conformation visible at 

 the anterior surface of the medulla oblongata, and known as the decus- 

 xation of the pyramids. After crossing from one side to the other, the 

 fibres again gradually take a longitudinal direction, and it is on this 

 account that the pyramids increase in size from below upward. The 

 pyramids accordingly consist of fibres which are derived from the lateral 

 and posterior columns of the opposite side of the cord. They are not 

 the continuation of the longitudinal tracts which form the anterior 

 columns below j these columns, on the contrary, diminishing rapidly in 

 size from within outward, until they disappear almost completely above 

 the level of the decussation of the pyramids. Thus the decussation of 

 the pyramids represents that of all the remaining longitudinal fibres of 

 the spinal cord, so far as it takes place in the medulla oblongata. 



The longitudinal fibres of the posterior columns also diminish con- 

 siderably in number in the medulla oblongata, as shown by Henle, 

 owing to their taking a horizontal direction, forward and inward, to 

 reach the decussation of the anterior pyramids ; while the remaining 

 longitudinal fibres of these columns pass into and through the restiform 

 bodies to the white substance of the cerebellum. 



The arrangement of the gray substance in the medulla oblongata is 

 also different from that presented in the spinal cord. In the first place, 

 according to the observations of Kolliker, it increases in quantity from 

 below upward. Secondly, the central mass of gray substance, which in 

 the cord surrounds the central canal, and sends out on each side the 

 anterior and posterior horns, recedes in the medulla oblongata farther 

 and farther backward ; the posterior horns spreading out laterally, and 

 the remainder occupying the space between them. The posterior 

 median fissure also becomes gradually shallower and wider by the 

 divergence of the posterior columns ; and the central canal approxi- 

 mates the posterior wall of the medulla, finally opening upon its surface 

 at the lower part of the fourth ventricle. The gray substance of the 

 medulla oblongata is thus uncovered posteriorly, and forms a layer 

 spread out laterally, on each side of the median line, immediately be- 

 neath the floor of the fourth ventricle. It extends forward, without any 

 complete interruption, beneath the whole length of the fourth ventricle 

 and the aqueduct of Sylvius ; and it is this layer of gray substance 

 which gives origin, at various points, to the fibres of all the cranial 

 nerves, excepting the olfactory and the optic. 



Thirdly, the medulla oblongata is distinguished by the appearance, 

 in its interior, of other deposits, or nuclei, of gray substance, detached 



