492 THE XEE.VOUS SYSTEM. 



sively ; nor is it a mere trunk, like a great nerve, through 

 which they may pass to the brain. It is, indeed, among 

 the most difficult things in structural anatomy to determine 

 the course of individual nerve-fibres, or even of fasciculi 

 of fibres, through even a short distance of the spinal cord ; 

 and it is only by the examination of transverse and longi- 

 tudinal sections through the substance of the cord, such as 

 those so successfully made by Mr. Lockhart Clarke, that we 

 can obtain anything like a correct idea of the direction 

 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 

 iisually first traverse a portion of the grey substance, some 

 pass upwards, and others, at least of the posterior roots, 

 turn downwards, but how far they proceed in either direc- 

 tion, 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, pro- 

 bably, 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 sizo 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 



