660 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



to mark the boundary between them; and although the separation of the 

 Medulla Spinalis from the Medulla Oblongata which is thus established, 

 is in itself purely artificial, yet it will be found to correspond completely 

 with the natural division founded on their respective physiological attributes. 

 The Spinal Cord* then, which extends from the margin of the foramen 

 magnum to the first or second lumbar vertebra, and which is prolonged as 

 thefilum terminale* to the extremity of the sacral canal, is almost completely 

 divided by the anterior and posterior median fissures (Fig. 174), into two 

 lateral and symmetrical halves. The "anterior median fissure" (a) is more 

 distinct than the posterior, being wider at the surface ; but it only pene- 

 trates to about one-third of the thickness of the Cord, its depth increasing, how- 

 ever, towards its lower part. The sides of the " posterior median fissure" (p), 

 on the other hand, are in closer approximation ; but the division commonly ex- 

 tends to about half the thickness of the cord, being deeper towards its upper 

 than towards its lower end. The two halves, therefore, are only united by a 

 commissural band which occupies the central part of the cord, and this is 

 traversed by the " Spinal canal" (/), which is continued downwards from 

 the fourth ventricle. 3 At a little distance from either side of the posterior 

 median fissure, and corresponding with the line of attachment of the posterior 

 roots of the nerves, is the posterior lateral furrow ; a shallow, longitudinal de- 

 pression, which marks out the " posterior columns" of the cord as distinct from 

 the " antero-lateral columns." A corresponding furrow has been sometimes 

 described as traversing the Cord in the line of the anterior roots of the nerves 

 on either side ; but this can scarcely be said to have a real existence ; and the 

 separation of the "antero-lateral columns" into the ts anterior" and the " lateral" 

 columns, is only marked externally by the attachment of the nerve-roots, but 

 is made more obvious internally by the peculiar distribution of the gray matter. 

 These columns are entirely composed of nerve-fibres, whose general direction is 

 longitudinal; and of these fibres it is quite certain that some are directly con- 

 tinuous with those which constitute the roots of the spinal nerves. It has been 

 generally considered by Anatomists, that the anterior roots are chiefly connected 

 with the anterior columns, but that some of their fibres are also continuous with 

 the lateral columns; and that the posterior roots are in like manner connected 

 chiefly with the posterior, but also with the lateral columns. Some have main- 

 tained, on the other hand, that both sets of nerve-roots are connected with the 

 lateral columns exclusively. According to the most recent researches, however, 

 it appears that the anterior roots are directly connected with the anterior columns 

 only, and the posterior roots with the posterior columns alone ; but of those 

 fibres from both these roots which pass at first into the gray matter, a certain 

 proportion emerge from this and enter the lateral columns. To what extent 

 any of these fibres proceed along the columns of the Spinal Cord, no precise 

 anatomical evidence has yet been obtained ; and although there are appearances 

 which may be regarded as sanctioning the idea that a direct and continuous 

 communication is established by their means between the roots of the nerves 



1 The sketch given in the text of the anatomy of the Spinal Cord is chiefly derived from the 

 statements of Prof. Kolliker in his "Mikroskopische Anatomic" (Band ii. \\ 115, 116), and 

 of Mr. J. L. Clarke in the "Philosophical Transactions," 1851 ; between which there is a 

 general accordance. 



2 The structure of the " filum terminate" is in every respect essentially the same as that 

 of the proper Spinal Cord, save that no nerve-roots are connected with it. 



3 The spinal canal is much more obvious in Fishes ; and the commissural connection be- 

 tween the two halves of their spinal cord is far less distinct than in higher Vertebrata. The 

 canal can only be distinguished in Man, being no more than l-100th of an inch in diameter, 

 by submitting thin transverse sections of the Cord to microscopic examination. 



