280 



PHYSIOLOGY 



CHAP. 



issuing from it by two distinct roots. The ventral root consists of 

 a larger number of slender bundles ; the dorsal root contains a 

 smaller number of thicker bundles. The roots on the two sides 

 are never perfectly symmetrical. The ventral roots (excepting 

 those of the first cervical pair) are as a whole smaller than the 

 dorsal roots and probably contain a smaller number of fibres. 

 This was in fact determined by Birge on two frogs, in which the 

 dorsal roots contained, respectively, 3781 and 5335 fibres, and the 

 ventral roots 3528 and 4283 fibres. 



Fio. 168. Different views of a portion of the spinal cord from the cervical region with the roots of 

 the nerves. Slightly enlarged. (Allen Thomson.) In A the anterior or ventral surface is 

 shown, the ventral nerve-root of the right side having been divided ; B, view of the right side ; 

 C, the upper surface ; D, nerve-roots and ganglion from below. 1, ventral median fissure ; 

 2, dorsal median fissure ; 3, ventro - lateral impression, over which the bundles of the 

 ventral nerve-root are seen to spread (too distinct in figure) ; 4, dorso-lateral groove, into which 

 *the bundles of the dorsal root are seen to sink ; 5, ventral root ; 5', in A, ventral root divided 

 and turned upwards ; (5, dorsal root, the fibres of which pass into the ganglion, 6' ; 7, united 

 or compound nerve ; 7', dorsal primary branch, seen in A and D to be derived partly from 

 ventral, partly from dorsal root. 



In vertebrates the length of the individual segments of the 

 spinal cord is not, as a rule, equal to the height of the correspond- 

 ing vertebrae ; it usually decreases from above downwards, so that 

 the length of the spinal cord only amounts to three-quarters that 

 of the vertebral canal. The successive roots in descending series 

 have therefore a more oblique longitudinal course, and travel 

 farther before they reach the corresponding intervertebral foramina. 

 The lower part of the vertebral canal merely contains a mass of 

 nerve-roots known as the cauda equina. 



The cervical enlargement of the cord, which comprises the 

 region of the roots that make up the braehial plexus, is largest at 



