334 THE NERVOUS SYSTEM 



rectum, colon and bladder, and inhibition of the neck of 

 the bladder and internal sphincter ani. 



INTEROCEPTIVE OR VISCERAL SENSATION 



Compared with the exteroceptive field the interoceptive 

 field possesses very few afferent nerves, and there are no 

 sensory endings, free nerve-endings, or touch-spots such 

 as are found in the skin. 



Touch. — The whole of the mucous membrane of the 

 alimentary canal, from the upper end of the oesophagus 

 to the lower end of the rectum, is insensitive to touch. 



Temperature. — The oesophagus and anal canal are sen- 

 sitive to temperature, the former, hke the mouth, being 

 ■able to withstand a higher temperature than the skin. 

 The stomach is usually insensitive to temperature, sensa- 

 tions of temperature commonly regarded as arising in 

 the stomach being in reahty felt at the lower end of the 

 oesophagus. If two concentric tubes, one within the 

 other, be passed into the stomach and water poured down 

 the inner one, the subject is usually unable to tell whether 

 the water is hot or cold. The intestine similarly is 

 insensitive to temperature, and the colon and rectum 

 usually so. 



Chemicals. — Both the oesophagus and stomach are insen- 

 sitive to dilute acids. Alcohol of 50 per cent, and glycerine 

 cause in the stomach a burning sensation. 



Pain. — The stomach and intestines, gall bladder, bile 

 ducts and ureter are completely insensitive to pin-pricks, 

 cuts and pinching. Sensation of pain only arises as the 

 result of abnormal tension of the muscle-fibres. This 

 occurs when a part of the viscus goes into spasmodic 

 contraction, e.g. upon an obstruction such as a gall-stone 

 or renal calculus, or when there is obstruction to the 

 normal peristaltic wave of the stomach or intestine. 



The sensation of fullness is caused by a mild degree of 

 muscular tension. 



