488 HANDBOOK OF PHYSIOLOGY. 



down stairs, or to form each letter or word in writing by a distinct exer- 

 cise of the will. 



These actions, however, will be again referred to, when treating of 

 their possible connection with the functions of the Sensory Ganglia. 



Morbid reflex actions. The relation of the reflex action to the 

 strength of the stimulus is the same as was shown generally in the action 

 of ganglia, a slight stimulus producing a slight movement, and a greater, 

 a greater movement, and so on; but in instances in which we must as- 

 sume that the cord is morbidly more irritable, i. e., apt to issue more 

 nervous force than is proportionate to the stimulus applied to it, a slight 

 impression on a sensory nerve produces extensive reflex movements. 

 This appears to be the condition in tetanus, in which a slight touch on 

 the skin may throw the whole body into convulsion. A similar state is 

 induced by the introduction of strychnia, and, in frogs, of opium, into 

 the blood; and numerous experiments on frogs thus made tetanic, have 

 shown that the tetanus is wholly unconnected with the brain, and de- 

 pends on the state induced in the spinal cord. 



Special Centres in Spinal Cord. 



It may seem to have been implied that the spinal cord as a single 

 nerve-centre, reflects alike from all parts all the impressions conducted 

 to it. This, however, is not the case, and it should be regarded as we 

 have indicated, as a collection of nervous centres united in a continuous 

 column. This is well illustrated by the fact that segments of the cord 

 may act as distinct nerve-centres, and excite muscular action in the 

 parts supplied with nerves given off from them; as well as by the anal- 

 ogy of certain cases in which the muscular movements of single organs 

 are under the control of certain circumscribed portions of the cord. The 

 special centres are the following: 



(a.) Centre for Defcecation, or Ano-Spinal centre. The mode of ac- 

 tion of the ano-spinal centre appears to be this. The mucous membrane 

 of the rectum is stimulated by the presence of faeces or gases in the 

 bowel. The stimulus passes up by the afferent nerves of the haemor- 

 rhoidal and inferior mesenteric plexus to the centre in the cord, situated 

 in the lumbar enlargement, and is reflected through the pudendal plexus 

 to the anal sphincter on the one hand, and on the other to the mus- 

 cular tissue in the wall of the lower bowel. In this way is produced a 

 relaxation of the first and a contraction of the second, and expulsion of 

 the contents of the bowel follows. The centre in the spinal cord is par- 

 tially under the control of the will, so that its action may be either in- 

 hibited, or augmented or helped. The action may be helped by the 

 abdominal muscles which are under the control of the will, although 

 under a strong stimulus they may also be compelled to contract by reflex 

 action. 



