284 THE DIGESTIVE SYSTEM 



The lower part of the anal canal is lined by modified skin. 

 This difference in structure indicates a difference in develop- 

 mental origin (p. 287), and is characterised by a difference in 

 nerve-supply. Thus, the upper part of the anal canal is 

 supplied through the sympathetic system, while the lower part 

 is supplied by the pudendal (internal pudic) nerve (p. 183). 



The Hsemorrhoidal Venous Plexus is situated in the sub- 

 mucous tissue of the anal canal. The importance of this 

 plexus depends on the fact that it constitutes a free communica- 

 tion between the superior haemorrhoidal vein, which passes, 

 via the inferior mesenteric and the splenic, to the portal vein, 

 and the middle and inferior haemorrhoidal veins, which pass 

 via the hypogastric (internal iliac) and common iliac veins to 

 the inferior vena cava. 



The superior hczmorrhoidal vein ascends for some distance in 

 the submucous tissue of the rectum before it pierces the 

 muscular wall of the gut, and it is, therefore, subjected to 

 compression during defaecation. In portal obstruction (p. 274) 

 or in chronic constipation, the blood is dammed back in the 

 superior haemorrhoidal vein and the pressure in the haemor- 

 rhoidal plexus is greatly increased. As the submucous tissue 

 in which they lie is very disiensible, the veins of the plexus 

 become varicose and constitute the condition known as 

 internal haemorrhoids. 



Irrigation of the Large Intestine is frequently necessary in 

 children suffering from epidemic enteritis, and it is important 

 that the gut should not be overdistended by the introduction 

 of more fluid than it can contain without dilating. Holt 

 estimates that at six months the colon will hold not more than 

 i pint, while at two years 2! to 3 pints can be introduced 

 without distending the gut. 



The NERVE-SUPPLY OF THE LARGE INTESTINE. The large 

 intestine, from the caecum to the pelvic colon, inclusive, is 

 supplied by sympathetic nerves which have their centres 

 situated in the lower thoracic and upper lumbar segments of 

 the spinal medulla. The rectum receives some fibres from the 



