416 STUDIES IN ADVANCED PHYSIOLOGY. 



the cord, running for some distance laterally from the cord 

 as separate trunks, but before leaving the dura-mater, unit- 

 ing on each side to form a single spinal nerve. On the 

 posterior root of the spinal nerve just previous to its union 

 with the anterior root, occurs a ganglion called for evident 

 reasons the posterior spinal root ganglion. The physiolog- 

 ical significance of these roots need not concern us here, 

 save the preliminary statement that the fibers which leave 

 the cord from the anterior horn are motor in their nature ; 

 that is, carry impulses outward towards the muscles, while 

 the fibers entering at the posterior horn are sensory fibers 

 carrying sensations inward to the cord and brain. 



Immediately after the formation of the spinal nerve by 

 the union of sensory and motor trunks it divides into three 

 branches, a posterior primary, distributed mainly to the 

 skin and muscles of the back, an anterior primary, giving 

 off nerves for the sides and ventral portion of the trunk 

 and for the limbs, and a third communicating branch which 

 runs to the neighboring sympathetic ganglion. It is through 

 this connecting branch that the co-ordination and subordi- 

 nation of the sympathetic system is effected. 



Thirty-one pairs of such spinal nerve trunks arise from 

 the spinal cord, leaving the neural canal through the inter- 

 vertebral foramina. Each of these spinal nerves has a 

 specific name, the name being derived from the vertebra 

 situated immediately in front of it. Thus, the nerve that 

 arises between the second and third thoracic vertebrae is the 

 second thoracic spinal nerve ; that which arises between the 

 fourth and fifth lumbar vertebrae is the fourth lumbar nerve. 

 The spinal nerves which arise in the lower lumbar, sacral 

 and coccygeal region run down through the neural canal, 

 occupying the space in this portion which the cord has 

 further up, the cord being here reduced to the filum ter- 

 minale. This big bunch of nerves is called the horse's 

 tail, or cauda eq^l^na. 



