406 ANATOMY AND PHYSIOLOGY OF THE SPINAL COED. 



sudden withdrawal of the limb may be still brought 

 about. 



The excitability of the cord, as exemplified in reflex 

 actions, may be morbidly increased or diminished. In 

 certain states of disease, as tetanus, or in poisoning by 

 strychnia, it becomes so susceptible that the slightest sound 

 or touch will often throw the whole voluntary muscular 

 system into violent spasmodic action. 



This reflex action is -highly important as a protecting 

 agent in case of irritants brought into contact with the body, 

 but it is an agent in many other functions of a no less 

 important character. The sphincter muscles of the bladder 

 and anus are entirely dependent on a constant reflex 

 action to keep them closed. Accordingly, when injury or 

 disease has destroyed the functions of the lower part of the 

 spinal cord, these relax, and the contents of the bowels and 

 bladder escape involuntarily, and it may be without the 

 knowledge of the patient. 



The dilator muscle of the iris is under the control of that 

 part of the cord which corresponds to the second dorsal 

 nerve. Under the stimulation of the posterior columns or 

 sensory roots at this part, the muscle contracts, and a 

 marked dilatation of the pupil is effected. The transmission 

 takes place through the sympathetic nerve. 



By reflex actions the spinal cord seems to control, to some 

 extent, the movements of the intestines, nutrition, and 

 animal heat. Thus organic diseases of the cord are always 

 accompanied by obstinate constipation, evidently dependent 

 principally on the absence of the muscular movements of 

 the intestines. The wasting of paralyzed parts may probably 

 have a similar origin, since the circulation, and necessarily 

 the nutrition, is greatly favoured by the muscular move- 

 ments. Lastly, the heat of the body is largely dependent 



