494 THE NERVOUS SYSTEM. 



taken by the fibres of the roots of the spinal nerves within 

 the cord. From the information afforded by such sections 

 it would appear, that of the root-fibres of the nerve which 

 enter the cord, some assume a transverse, others a longi- 

 tudinal direction : the fibres of the former pass horizontally 

 or obliquely into the substance of the cord, in which many 

 of them appear to become continuous with fibres entering 

 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 grey sub- 

 stance, some pass upwards, and others, at least of the 

 posterior roots, turn downwards, 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, connect- 

 ing 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 forma- 

 tion 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 lumbar, at the 

 origins of the large nerves which, after forming the lum- 

 bar and sacral plexuses, are distributed to the lower 

 extremities. The chief cause of the greater size at these 

 parts of the spinal cord is increase in the quantity of grey 

 matter ; for there seems reason to believe that the white 

 or fibrous part of the cord becomes gradually and pro- 

 gressively larger from below upwards, doubtless from the 

 addition of a certain number of upward passing fibres from 

 each pair of nerves. 



