Dr. Foville on the Anatomy of the Brain. 17 



all the parts of the brain, which being altered produce sym- 

 ptoms on the opposite side. Hence in our survey the necessity 

 of tracing accurately the connexions between the pyramids 

 and the other parts of the cerebro-spinal system. 



The spinal marrow situated below the decussation of the 

 pyramids is composed of two symmetrical portions, which are 

 united by a commissure of white matter. Without entering 

 into the consideration of how many distinct columns are to be 

 found in each half, I will now point out the different appear- 

 ances belonging to the anterior and posterior surfaces of its 

 commissure. 



Throughout its whole extent the anterior commissure is 

 divided longitudinally by numerous small holes, which are 

 never to be found absolutely on the median line, where there 

 exists a kind of very small raphe. At its superior extremity 

 this anterior surface of the commissure becomes more super- 

 ficial, and gradually assumes something of the aspect belong- 

 ing to the decussation of the pyramids; so much so, that it is 

 not always easy to determine where the commissure ends 

 and the decussation begins. 



The posterior surface of this commissure, which is visible 

 at the bottom of the posterior fissure, does not present the 

 same appearance as the anterior surface. The two surfaces 

 are so near each other that they seem at first to belong to the 

 same fibrous layer; but a close examination demonstrates 

 that this is not the case, for there is constantly a sensible in- 

 terval between the two surfaces. On the median line, two 

 small very fine parallel white layers pass from the one to 

 the other. If this double commissure be compared to the 

 corpus callosum and fornix, these white layers going from the 

 one to the other bear some analogy to the septum luciclum in 

 the brain. 



I think this cavity has already been noticed by Malpighi ; 

 and Dr. Hodgkin believes that he has more than once ob- 

 served it. Gall described a longitudinal cavity on each side of 

 the spinal cord, but it has been ascertained that they were pro- 

 duced by blowing into the central longitudinal masses on each 

 side of the marrow. Thus we find in the commissure two 

 transverse bands of white matter uniting the two halves of the 

 spinal marrow, which bands are connected together on the 

 median line by a delicate double band of white matter be- 

 tween the layers, of which a .natural cavity seems to exist. 



Let us now inquire what becomes of the two commissures 

 when they penetrate on each side into the substance of the 

 marrow. It seems to me that they soon become united to- 

 gether so as to constitute a kind of axis, around which are 

 attached the lateral fibrous columns by means of fibres ra- 



Phil. Mag. S. 3. Vol. 17. No. 107. July 18*0. C 



