124 MANUAL OF PHYSIOLOGY. 



However, most of these effects may be explained by referring 

 them to vaso-motor changes. 



With regard to defecation, we know that a nerve centre exists 

 in the lumbar portion of the spinal cord, which governs the 

 sphincter, and seems to keep up its tonic contraction. This 

 centre may be either excited to increased action or inhibited, by 

 peripheral stimuli or by central influences from the brain. 



Thus the local application of warmth causes inhibition of the 

 centre, and thereby relaxation of the sphincter, while cold gives 

 rise to increased central action, causing contraction of the sphinc- 

 ter muscle (a point to be remembered when examining or op- 

 erating within its grasp). Besides the voluntary variations which 

 we can bring about in the activity of this lumbar centre, many 

 other central influences, such as emotions, may operate upon it. 

 Thus terror inhibits the centre, and loosens the sphincter inde- 

 pendently of our will. 



