700 



THE NERVOUS SYSTEM. 



describe the appearances as they are seen in the upper and lower portions of the 

 medulla, since they differ considerably in these two parts. 



The Lower Part of the Medulla. The first change in the internal structure is 

 caused by the passage of the fibres of the crossed pyramidal tract obliquely 

 through the gray matter of the anterior horn. As stated above, the pyramid is 

 composed of fibres derived from the direct pyramidal tract of the anterior column 

 of the cord of the same side, and from the crossed pyramidal tract of the lateral 

 column of the opposite half of the cord. Those fibres which are derived from the 

 direct pyramidal tract and which in the cord lie close to the median fissure are in 

 the medulla placed to the outer side of the pyramid, being pushed aside, as it 

 were, by the interpolation of the fibres derived from the crossed pyramidal tract, 

 which are much more numerous. The crossed pyramidal fibres ascend from the 

 lateral column of the spinal cord, and, passing through the anterior gray cornu 

 and across the middle line, form the inner part of the pyramid. In consequence 

 of this passage of white fibres through its substance the anterior gray cornu is 



Nucleus offuniculus teres. 



Nucleus ambig. - ^ 



Raphe. ^ 



For mat io reticularis-. -| 



Vagus nuclei 

 Hypoglossal 



Ligula. 



Continuation of antero _ 

 lateral ground-bundle = 



Accessory olivary nuclei*'- - 



Funiculus Solitarius. 

 Nucleus gracilis. 



^Nucleus cuneatus. 



Corpus restiforme. 



Nucleus of Rolando 



'Ascending root of 

 fifth nerve. 



Vagus root. 



rciform fibres. 

 Nucleus lateralis. 



Olivary nucleus. 



poglossal nerve. 

 Nucleus of ext. arc. fibres. 

 "External arciform fibres. 

 Anterior median fissure. 



FIG. 372. Section of the medulla oblongata at about the middle of the olivary body. (Schwalbe.) 



broken up into a coarse network, while one portion of it, the caput cornu, is 

 entirely separated from the rest ; only the base of the cornu remains intact, close 

 to the ventro-lateral aspect of the central canal. The caput cornu, thus separated, 

 is displaced laterally, and comes to lie close to the caput cornu posterioris, which 

 has also shifted its position. In consequence of this breaking up of the greater 

 part of the anterior gray cornu by white fibres a coarse network is formed in the 

 anterior and lateral areas of the medulla, which is named the formatio reticu- 

 laris. 



The posterior cornu also undergoes somewhat similar changes. It becomes 

 subdivided by the passage through it of the sensory fibres of the columns of Goll 

 and Burdach. These pass across to the opposite anterior area of the medulla, 

 where they are seen to lie immediately on the dorsal aspect of the pyramids. In 

 their passage through the posterior horns of gray matter the latter become sub- 

 divided, in a manner somewhat similar to what has been seen to occur in the 

 anterior horns. This crossing of the sensory fibres is termed the superior pyram- 



