572 HANDBOOK OF PHYSIOLOGY. 



as well as producing muscular movemeDts: the pain thus excited is prob- 

 ably the result either of cramp or of so-called recurrent sensibility. 



Recurrent Sensibility. If the anterior root of a spinal nerve be 

 divided, and the peripheral end be irritated, not only movements of the 

 muscles supplied by the nerve take place, but also of other muscles, indic- 

 ative of pain. If the main trunk of the nerve (after the coalescence of 

 the roots beyond the ganglion) be divided, and the anterior root be 

 irritated as before, the general signs of pain still remain, although the 

 contraction of the muscles does not occur. The signs of pain disappear 

 when the posterior root is divided. From these experiments it is be- 

 lieved that the stimulus passes down the anterior root to the mixed 

 nerve, and returns to the central nervous system through the posterior 

 root by means of certain sensory fibres from the posterior root, which 

 loop back into the anterior root before continuing their course into the 

 mixed nerve-trunk. These fibres degenerate when the posterior nerve- 

 root is divided beyond the ganglion. 



Functions of the Ganglia on Posterior Roots. The cells of the pos- 

 terior ganglia act as centres for the nutrition of the nerve-fibres given off 

 from them. When these are cut, the parts of the nerves so severed de- 

 generate, while the parts which remain in connection with the cells do 

 not. Thus on section of the posterior nerve-root beyond the ganglion 

 the peripheral part wastes and the central does not, and on section of 

 the root between the ganglion and the cord the central part to a great 

 extent wastes and the peripheral remains unaffected. 



FUNCTIONS OF THE SPINAL CORD. 



The power of the spinal cord, as a nerve-centre, may be arranged 

 tinder the heads of (1) Conduction; (2) Reflex action. 



(1) Conduction. The functions of the spinal cord in relation to 

 conduction may be best remembered by considering its anatomical con- 

 nections with other parts of the body. From these it is evident that 

 there is no way by. which nerve-impulses can be conveyed from the trunk 

 and extremities to the brain, or vice versa, other than that formed by 

 the spinal cord. Through it, the impressions made upon the peripheral 

 extremities or other parts of the spinal sensory nerves are conducted to 

 the brain, where alone they can be perceived. Through it, also, the 

 stimulus of the will, conducted from the brain, is capable of exciting the 

 action of the muscles supplied from it with motor nerves. And for all 

 these conductions of impressions to and fro between the brain and the 

 spinal nerves, the perfect state of the cord is necessary; for when any 

 part of it is destroyed, and its communication with the brain is inter- 

 rupted,' impressions on the sensory nerves given off from it below the 



