SECTION VII 

 THE SPINAL CORD AS A REFLEX CENTRE 



IN the evolution of the cord the primitive segmental arrangement has been 

 especially interfered with by the development of the four limbs. Since 

 the reactions of the limbs transcend in importance and complexity those of 

 the rest of the body, a great enlargement of the cord has occurred in the region 

 of the nerve-roots which supply the limbs. Each limb must be considered 

 as produced by the fusion of a number of body segments, in which the 

 morphological segmental arrangement has entirely given place to a physio- 

 logical one. Thus no single muscle of the limbs is innervated from one 

 nerve-root, every muscle being formed from elements belonging to several 

 segments and innervated from several nerve-roots. The segmental arrange- 

 ment of the cord is hidden moreover by the increasing complexity of the spinal 

 reflexes and the consequent involvement of many segments in even the 

 simplest reactions. __As_we shall see later, practically no reflex can be 

 evoked, even by stimulation of one nerve fibre or nerve-root in any of the 

 vertebra ta, which does not involve in its response elements belonging to many 

 segments. 



Since the reactions, which can be carried out by any part of the nervous 

 system, depend on the neurons of which the part is composed, it is necessary, 

 before treating of the reactions of the spinal animal, to consider the " way in " 

 to and the " way out " of the centre, as well as the connections between the 

 entering and issuing paths. Each segment of the cord gives off a pair of 

 nerve-roots, subdivided into an anterior and a posterior root (Fig. 158). In 

 mammals it is easy to show that the posterior root is exclusively afferent in 

 function. Section of the root, either distal or proximal to the ganglion, pro- 

 duces no paralysis of any description. It may cause diminished sensation in 

 the area supplied by it, and if two or three adjacent posterior roots be divided, 

 complete anaesthesia results in the central part of the skin area supplied from 

 these roots. Stimulation of the central end of a divided posterior root 

 evokes in a conscious animal signs of pain. In an animal possessing only 

 spinal cord and bulb, reflex effects are produced, i.e. movements of skeletal 

 muscles as well as effects on visceral muscles, such as constriction of blood- 

 vessels, relaxation of intestinal muscle, and so on. On the other hand, 

 section of an anterior root causes paralysis of muscles or parts of muscles. 

 Section of all the anterior roots going to a limb will produce complete 



motor paralysis of the limb. Stimulation of the central end of a divided 

 ^ 322 



