THE PROPRIO-CEPTIVE MECHANISM 



661 



The call to defecation is due to distension of the rectum, and 

 may be evoked artificially, e.g., by the introduction of fluid, or of a 

 balloon and its distension to a pressure of about 50 mm. Hg 

 (see p. 419). 



Pain is due to the stretching of the alimentary tract by obstruction 

 or overdistension. Colic is due to a tonic contraction of the gut, 

 which prevents peristalsis from forcing on the contents of the gut. 

 Vague sensations, such as uneasiness, tingling, tickling, are due to 

 some form of abnormal stimulation 

 As a rule, pain in the alimentary tract 

 is not well localized, but is more 

 accurately localized in the fixed than 

 in the movable viscera. The nerves 

 subserving the, pain sensation are the 

 sympathetic, and the pain is referred 

 to the superficial cutaneous areas 

 supplied by those nerves which are in 

 connection with the same segment of 

 the cord as that from which the sympa- 

 thetic supply of the viscus is derived. 

 The stimulation of the sympathetic 

 nerves produces an irritable focus in 

 the cord, and this gives rise to hyper- 

 sensitivity and so to pain felt in the 

 peripheral tissues connected with that 

 segment of the cord. In the case of 

 the stomach, the hypersensitivity 

 affects the seventh, eighth, and ninth 

 thoracic nerves; of the intestine, the 

 tenth and eleventh thoracic; of the 

 rectum, the first, second, and third 

 sacral. An irritant applied to the 



skin in the area where the pain is felt may take possession of the 

 sensory nerve and the attention of the patient so that those from 

 the viscus have no effect. 



FlG. 387. DIAGRAM TO EXPLAIN 

 REFERRED FAIN AND COUNTER- 

 rRRiTATioN. (Dixon, after 

 Mackenzie.) 



The diagram shows, also, how 

 irritants to the skin may cause 

 local dilatation- of the vessels 

 bv an axon reflex. 



