98 VETERINARY PHYSIOLOGY 



the wall of the alimentary canal where the co-ordinated 

 reflex of peristalsis (p, 333), involving co-ordinated con- 

 traction behind the contents of the gut and inhibition in 

 front of the contraction, is dominated by the myenteric 

 plexus. The peripheral mechanism is controlled by impulses 

 from the central nervous system passing by excitor and by 

 inhibitory nerves which are reflexly called into play when 

 any modification of the ordinary peristalsis is required. 

 The importance of the local mechanism is indicated by the 

 small number of nerve fibres which pass to the spinal cord 

 from the viscera. 



When the visceral receptors are abnormally stimulated, 

 abnormal reflex responses may result. Thus, when the 

 stomach and certain other parts of the visceral tract are 

 irritated, the act of vomiting may be produced. 



Normally the visceral reflexes, peripheral and central, are 

 carried out without consciousness being involved. But, in 

 abnormal, conditions, consciousness may be implicated and 

 sensations may be produced which direct the attention to the 

 condition of the viscera. This is sometimes called common 

 sensibility. 



In the stomach and intestine these sensations are generally 

 the result of stretching of the muscular coats. This may 

 lead to a feeling of distension or to actual pain. Later, the 

 existence of a separate set of pain receptors and pain nerves 

 in the skin will be considered. In the viscera there is no 

 evidence of their existence, and the unpleasant sensation 

 characterised as painful must be considered as due to over- 

 stimulation of nerves which, when normally stimulated, give 

 rise to no sensation. But there is evidence that abnormal 

 stimulation of visceral nerves may give rise to pain, which 

 is referred by the sufferer to the distribution of the corre- 

 sponding somatic nerve, such referred pain is seen in heart 

 disease (p. 423). The stomach and intestines seem to be 

 destitute of receptors capable of stimulation by touching, 

 pricking, or by changes of temperature. Hence, after the 

 abdominal wall is divided, abdominal operations may 

 frequently be performed without a general anaesthetic. 



The gullet and anal canal are provided with more 



