188 



ELEMENTARY PHYSIOLOGY 



root. These two roots unite to form a spinal nerve trunk. The 

 nerve trunks are thus formed by the union of anterior and pos- 

 terior roots ; and after division and subdivision they supply fibres 

 to the skin and the voluntary muscles. 



Each of the posterior roots has a ganglion, the use of which is 

 unknown. 



The functions of the roots of the spinal nerves have been 

 ascertained by the following observations : i. If the anterior 



Fig. J73. Boots of a Spinal Nerve issuing from the Cord. 



A, from before ; B, from the side ; C, from above ; D, the roots separated. 



i. anterior fissure ; 2, posterior fissure ; 3 and 4, lateral grooves of the cord ; 5, anterior root ; 

 6, posterior root ; 6', posterior ganglion ; 7, the united or compound nerve ; f, the posterior 

 branch. In A one anterior root is divided and turned upwards. 



roots of the spinal nerves supplying a certain limb are injured, 

 the power of voluntary movement in that limb will be more or 

 less destroyed, and the limb will hang in a flaccid condition, but 

 sensation will remain unimpaired. If one of the roots is divided, 

 any irritation applied to that portion which remains in contact 

 with the cord will cause no sensation, nor will there be any 

 movement of the limb in question. But if the other portion of 

 the divided root be iiritated, the muscles of the limb will .contract. 



