272 THE ANIMAL PARASITES OF MAN 



Distribution. In order to understand the distribution of the worms 

 and eggs in the body, it may be well to recall the blood supply of the 

 abdominal and pelvic organs. It is generally assumed that the early 

 life (?cercarial stage) of the worms occurs in the liver, and that the 

 young worms travel from here, where they are invariably found, to 

 their various sites along the portal vein and its tributaries and so 

 against the blood stream. The tributaries of the portal vein are : 



(1) Superior mesenteric, the tributaries of which are : (a} the veins 

 of the small intestine ; (b) ileo-colic ; (c) right colic ; (d) middle colic ; 

 (e) right gastro-epiploic ; and (/) inferior pancreatic. By these paths 

 infection of the small intestine, ascending and transverse colon and 

 pancreas would occur. 



(2) Splenic. (Ova have been recorded by Symmers in the spleen.) 



(3) Inferior mesenteric, the tributaries of which are (a) superior 

 haemorrhoidal veins from the upper part of the haemorrhoidal plexus ; 

 (b) sigmoid veins from sigmoid flexure and lower portion of descending 

 colon ; (c) left colic vein draining descending colon. 



The superior haemorrhoidal veins form a rich plexus in the rectum, 

 and below this level in the upper and middle parts of the anal canal. 

 The plexus forms two networks, an internal plexus in the submucosa 

 and an external on the outer surface. The internal plexus opens at 

 the anal orifice into : (a) branches of the inferior haemorrhoidal vein 

 (from the pudic) ; (b) the external plexus. The external plexus gives 

 off : (a) inferior haemorrhoidal opening into internal pudic (of internal 

 iliac vein) ; (6) mid-haemorrhoidal into internal iliac or its branches ; 

 and (c) superior haemorrhoidal opening into inferior mesenteric. 

 The external plexus further communicates with the vesico-prostatic 

 plexus. The vesico-prostatic (vaginal) plexus opens into the vesical 

 veins, which drain into the interior iliac vein. This plexus also 

 receives afferents from the pudendal plexus, the chief tributary of 

 which is the dorsal vein of the penis. The pudendal plexus also 

 receives branches from the inferior pudic and the anterior surface of 

 the bladder. 



There is thus a communication between the portal vein and the 

 vena cava by means of these plexuses, viz., through the inferior and 

 middle haemorrhoidals, and by the inferior haemorrhoidals to the 

 bladder and thence by the vesical veins or the pudic to the caval 

 system (interior iliac). 



It is thus by the inferior mesenteric and its tributaries that the 

 worms reach the descending colon, rectum, anal canal, and eventually 

 the bladder, and in some cases the caval system. 



Before considering what is actually found post mortem in these 

 veins and the organs drained by them, we may further recall the fact 

 that the calibre of "medium" veins is 4 to 8 mm., "small" veins 



