ON THE NERVES IN GENERAL. 489 



it continue on the same side. Dissection, moreover, does not 

 show that the crossing takes place in all the fibres; so that the 

 opinion of those who think it only partial is the most proba- 

 ble. But, this exception aside, the crossing of the nerves is 

 far from being demonstrated. As much may be said of that of 

 the two sides of the brain and cerebellum, which has been ad- 

 mitted. The anterior pyramids alone present this disposition, 

 which explains how, in affections of the brain, the symptoms 

 manifest themselves on the opposite side of the spinal mar- 

 row: thus, when this last is divided beneath the place where 

 the crossing of the pyramids takes place, the symptoms appear 

 on the same side. 



Another question which has been agitated among anatomists, 

 is to know if the nerves unite on the median line by commis- 

 sures analogous to those which are found between the corres- 

 ponding sides of the brain and cerebellum. This reunion is 

 evident only in the pathetic nerves. The auditory nerves are 

 also sometimes united, at their origin, by white striae, which 

 spread over the bottom of the fourth ventricle; but these striae 

 are far from being constant, and are generally wanting in youth. 



Almost all the nerves have a deep origin from the gray sub- 

 stance, and not from the white, which covers this last, and 

 under which they only dip. In the spinal marrow the nerves, 

 on being torn up, leave a pit, which shows that they do not 

 stop at the surface; and when the spinal marrow is hardened, 

 the roots of the nerves may be followed and seen traversing 

 the longitudinal fibres of this organ, to implant themselves in 

 the gray substance. In the cranium this disposition is also 

 evident as respects most of the nerves. The auditory alone 

 have their origin at the surface of the medulla oblongata; but 

 there exists at the same time the gray substance in the place 

 from which they spring: only this substance is superficially 

 placed; it forms the gray band. 



The nerves of the spinal marrow arise with two roots, one 

 anterior and one posterior, as has been said already. The re- 

 spective size of these two roots, upon which there have been 

 many different statements, and which Gall has said to be in 

 favour of the posterior root, is so really only in the bracJiial 

 nerves; the contrary takes place in the crural. These roots 



