390 THE NERVOUS SYSTEM. 



many of them appear to become continuous with fibres enter- 

 ing the cord from other roots; others pass into the columns of 

 the cord, while some perhaps terminate at or near the part 

 which they enter : of the fibres of the second set, which usually 

 first traverse a portion of the gray substance, some pass up- 

 wards, and others, at least of the posterior roots, turn down- 

 wards, but how far they proceed in either direction, or in what 

 manner they terminate, are questions still undetermined. It 

 is probable that of these latter, many constitute longitudinal 

 commissures, connecting different segments of the cord with 

 each other ; while others, probably, pass directly to the brain. 



The general rule respecting the size of different parts of the 

 cord appears to be, that the size of each part bears a direct 

 proportion to the size and number of nerve-roots given off from 

 itself, and has but little relation to the size or number of those 

 given off below it. Thus the cord is very large in the middle 

 and lower part of its cervical portion, whence arise the large 

 nerve-roots for the formation of the brachial plexuses and the 

 supply of the upper extremities, and again enlarges at the 

 lowest part of its dorsal portion and the upper part of its lum- 

 bar, at the origins of the large nerves, which, after forming the 

 lumbar and sacral plexuses, are distributed to the lower ex- 

 tremities. The chief cause of the greater size at these parts of 

 the spinal cord is increase in the quantity of gray matter ; for 

 there seems reason to believe that the white or fibrous part of 

 the cord becomes gradually and progressively larger from be- 

 low upwards, doubtless from the addition of a certain number 

 of upward-passing fibres from each pair of nerves. 



It may be added, however, that there is no sufficient evi- 

 dence for the supposition that an uninterrupted continuity of 

 nerve-fibres is essential to the conduction of impressions on the 

 spinal nerves to and from the brain : such impressions may be 

 as well transmitted through the nerve-vesicles of the cord as 

 by the nerve-fibres; and the experiments of Brown-Se'quard, 

 again to be alluded to, make it probable that the gray sub- 

 stance of the cord is the only channel through which sensitive 

 impressions are conveyed to the brain. 



The Nerves of the Spinal Cord consist of thirty-one pairs, 

 issuing from the sides of the whole length of the cord, their num- 

 ber corresponding with the intervertebral foramina through 

 which they pass. Each nerve arises by two roots, an anterior 

 and posterior, the latter being the larger. The roots emerge 

 through separate apertures of the sheath of dura mater sur- 

 rounding the cord ; and directly after their emergence, where 

 the roots He in the intervertebral foramen, a ganglion is found 



