SECTION VII 

 THE SPINAL CORD AS A REFLEX CENTRE 



IN the evolution of the cord the primitive segmental arrange- 

 ment 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 physiological 

 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 arrangement of the cord is hidden moreover by the 

 increasing complexity of the spinal reflexes and the consequent involve- 

 ment 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 vertebrata, 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. 159). 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, produces 

 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 



364 



