GRAY AND WHITE SUBSTANCE. 445 



The nerve fibres of the gray substance are, in general, of much 

 smaller diameter than those of the white substance, but otherwise pre- 

 sent the same anatomical characters. Most of them run in a horizontal 

 transverse, antero-posterior, or radiating direction. They consist, first, 

 of fibres which have penetrated into the gray substance from the ante- 

 rior and posterior roots of the spinal nerves; secondly, of fibres which 

 cross the median line in the gray commissure, from one side of the cord 

 to the other, thus forming a commissural connection between the two 

 lateral halves of the gray substance ; and, thirdly, of fibres which run 

 in a great variety of directions with regard to each other, and of which 

 the origin and terminations are unknown. 



The White Substance. The white substance of the spinal cord con- 

 sists of nerve fibres, the large majority of which run in a longitudinal 

 direction, forming tracts or "columns," designated, according to their 

 situation, by the names of the anterior, lateral, and posterior columns 

 of the cord. In transverse sections of the cord which have been prop- 

 erly hardened, the longitudinal fibres are readily distinguished by their 

 presenting the circular outline of a minute cylinder cut across ; while 

 those which are horizontal or oblique are seen in profile for a longer or 

 shorter distance in the preparation. 



The anterior, lateral, and posterior columns consist almost exclu- 

 sively of longitudinal fibres. But at the bottom of the anterior median 

 fissure there is a band of white substance, the fibres of which run hori- 

 zontally across from the inner portions of each anterior column to the 

 opposite side of the cord. This is called the white commissure of the 

 cord; but the name is not entirely appropriate, since its fibres do not 

 form a connection between exactly corresponding parts on the right and 

 left sides. According to both Kolliker and Gerlach, the fibres which 

 pass across at the median line from the right anterior column spread 

 out in the gray substance of the left anterior horn ; and those coming 

 from the left anterior column spread out in the gray substance of the 

 right anterior horn. The transverse fibres, accordingly, of the white 

 commissure connect the right and left anterior columns respectively 

 with the gray substance of the opposite side of the cord. 



Counting the transverse fibres, therefore, of both the gray and the 

 white commissures, it may be said that there is a direct bilateral con- 

 nection, by means of communicating nerve fibres, between the right 

 and left halves throughout the length of the cord ; but it is not possible 

 to determine, with precision, the exact origin or termination of the indi- 

 vidual fibres by which this connection is made. 



Connection of the Spinal Nerve-roots with the Spinal Cord. The 

 fibres of the anterior roots of the spinal nerves are distinguished from 

 those of the posterior by their relatively larger size ; but on penetrating 

 into the white substance of the cord, their diameter is diminished, and 

 they assume all the characters of the central nerve fibres. They then 

 pass inward, in a horizontal or oblique and backward direction, and 

 reach the gray substance of the anterior horn. 



