THE LARGE INTESTINE 285 



same source, but, like the urinary bladder (p. 371), it is also 

 supplied by sympathetic fibres which have their centres situ- 

 ated in the mid-sacral segments. The latter fibres constitute 

 the pelvic splanchnic* of Gaskell and they also supply the 

 upper part of the anal canal. On the other hand, the lower 

 part of the anal canal is supplied through the cerebro- 

 spinal system by the pudendal nerve (S. 2, 3 and 4). 



Violent peristalsis of the large intestine, such as occurs in 

 griping, gives rise to pain, which is often referred to the 

 peripheral distribution of the anterior rami (primary divisions) 

 of the eleventh and twelfth thoracic and the first lumbar 

 nerves, and especially to their anterior cutaneous branches 

 (Fig. 69). It is consequently experienced most acutely in 

 the hypogastric region, though it may also be referred to the 

 iliac regions and the lateral part of the buttock. 



In new growths or ulceration of the rectum, pain is frequently 

 referred to the perineum or to the back of the sacrum, and, in 

 rare cases, it is experienced in the back of the thigh. Reference 

 to Figs. 69 and 74 will show that these cutaneous areas are 

 supplied by spinal nerves which all arise from the same seg- 

 ments of the spinal medulla. Further, these segments also 

 give rise to the pelvic splanchnics, which supply the rectum. 



The extreme tenderness of the anal canal in the presence of 

 a fissure is purely local, for, since it is the lower part of the 

 canal which is abraded in this condition (p. 283), the sensory 

 branches of the pudendal nerve are directly stimulated. The 

 accompanying contraction of the sphincter ani externus, which 

 is also supplied by the pudendal nerve, is a good example of 

 the viscero-motor reflex. 



DEVELOPMENTAL ANOMALIES OF THE INTESTINAL CANAL. 

 Atresia ani and the presence of some form of Meckel's 

 diverticulum are the two commonest congenital abnormalities 

 met with in the alimentary canal. 



At an early stage the primitive alimentary canal consists of 

 a simple tube, closed at both extremities but open on its 

 ventral aspect, where it communicates with the yolk-sac 



