680 NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPHALON.) 



longata to some distance below the decussation, 

 and it is, in general, of advantage to the pre- 

 paration to place it in alcohol immediately after 

 the removal of the pia mater. 



A second set of fibres, very few in number, 

 are continued from the pyramids directly down 

 to the anterior surface of the cord on the same 

 side, and appear to be continuous with some 

 of the superficial fibres of the antero-lateral 

 column. These fibres may be regarded as 

 the direct channel of communication of each 

 half of the medulla oblongata with the corre- 

 sponding half of the spinal cord (jig. 385, ?<) 



The third series of fibres vary considerably 

 in point of developement in different indivi- 

 duals. They pass between the pyramids and 

 the postero-lateral columns of the medulla ob- 

 longata, the restiform columns. They form a 

 series of curves with their concavities directed 

 upwards (Jig. 383, A),crossing beneath the infe- 

 rior extremity of the olivary body, and some- 

 times extending over a considerable portion of 

 its surface. I have on several occasions seen 

 these fibres so largely developed as to cover 

 nearly the whole surface of each olivary body. 

 These fibres are appropriately distinguished by 

 the name arc i form from their arched course 

 (procesms arciformes, Santorini). 



When these fibres are so numerous as to 

 cover the surface of the olivary body, we may 

 observe that those which are nearest the margin 



Fig. 385. 

 p 



al 



Anterior view of the medulla oblongata, shewing the 

 decussation of the pyramids , and of the upper part 

 of the spinal cord. (After Mayo.) 



p, anterior pyramids. 

 o, olivary bodies. 

 r, restil'orm bodies. 

 d, decussating fibres. 



al, antero-lateral column of the spinal cord, 

 c, anterior fissure of the cord, the floor of which 

 forms the anterior commissure. 



sure. Superiorly the pyramids pass into the 

 mesocephale above the inferior fibres of the 

 ponsVarolii, and interlace with other fibres of 

 the same system which occupy a more elevated 

 plane. In its passage into the mesocephale, 

 each pyramid experiences a marked constric- 

 tion, which alters its form from a prism to a 

 cylinder. The fibres, however, soon diverge 

 and expand. As they ascend through the me- 

 socephale they are crossed by the transverse 

 fibres of the pons, and some grey matter occu- 

 pies the interstices between them, with which it is 

 probable that other fibres are connected, and are 

 added to those of the pyramids, as they emerge 

 from the mesocephale at its anterior extremity. 

 The pyramids gradually diminish in size 

 towards the inferior extremity of the medulla 

 oblongata. And here three sets of fibres may 

 be distinctly noticed. The first, or decussating 

 fibres, are the most numerous ; they pass 

 downwards and backwards into the antero- 

 lateral column of the spinal cord on the oppo- 

 site side, so that the right pyramid sends fibres 

 into the left half of the cord, and the left pyra- 

 mid into the riuht half of the cord. These decus- 

 sating fibres consist of from three to five bundles 

 from each pyramid, which in their descent cross 

 and interlace with each other (Jigs. 384, 385, d}. 

 They differ in distinctness as well as in number 

 in various subjects. The point at which the 

 decussation takes place is about ten lines below 

 the margin of the ponsVarolii, and the inter- 

 ruption to the fissure, occasioned by the cross- 

 ing of the fibres, occupies a space of from two 

 to four lines. To expose these fibres clearly it 

 is necessary to remove the pia mater carefully 

 from the anterior surface of the medulla ob- 



Anterior stir face of the medulla oblongata, with a por- 

 t inn a f the spinal cor A and of the pons Varolii, as seen 

 obliquely from the riyht side.. (After Mayo.) 



P, pons Varolii its left half. 



o, o, olivary bodies. 



p, part of the right anterior pyramid, cut across 

 near the inferior edge of the pons, and torn down, 

 showing the passage of some of its fibres over to 

 the left side and backwards. 



d, decussating fasciculus of fibres of right py- 

 ramid. 



d', decussating fasciculus of left pyramid. 



n, nou-decussuliug fibres of the right pyramid. 



