(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 (f) 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 hæmorrhoidal veins from the upper part of the hæmorrhoidal plexus; (b) sigmoid veins from sigmoid flexure and lower portion of descending colon; (c) left colic vein draining descending colon.
The superior hæmorrhoidal 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 hæmorrhoidal vein (from the pudic); (b) the external plexus. The external plexus gives off: (a) inferior hæmorrhoidal opening into internal pudic (of internal iliac vein); (b) mid-hæmorrhoidal into internal iliac or its branches; and (c) superior hæmorrhoidal 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 hæmorrhoidals, and by the inferior hæmorrhoidals 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 less than 40 µ in diameter and capillaries 8 µ to 20 µ. Further, the maximum diameter of the male worm is 1 mm., that of the female 280 µ and eggs in utero 80 µ to 90 µ long by 30 µ to 40 µ.
Liver and Portal Vein.—Here worms are most easily found post mortem. Often only males are found and these of the same size, and if females occur only a few worms are found in copulâ. The worms are frequently not full size and the males may contain no free spermatozoa in their testes, and as regards the females some may be fertilized, others not, as shown by the presence or absence of spermatozoa in the seminal receptacle or uterus. In either case they may contain eggs—lateral-spined—usually one, less often two, but there may be as many as five or six. These eggs may also show some abnormality, which takes the form of: (1) abnormal contents, viz., disintegrating yolk cells with or without an ovarian cell; (2) abnormal shape but with normal contents and probably represented by the collapsed and empty egg-shells which are found in the tissues.
As to the interpretation of these facts, Looss believes that these lateral-spined eggs are products of young females whose egg-laying is not at first properly regulated. The shape that the eggs take, viz., with a lateral spine, is determined by an excess of material—ovarian and yolk cells—being present in the oötype. The shape of eggs depends upon the position they have in the oötype during their formation. In young females an excess of cells—yolk cells especially—accumulates, distending not only the dorsal wall but a portion also of the short duct joining the oötype to the uterus. The result of this is that the axis of the oötype and egg is almost transverse to the body, and the posterior funnel-shaped portion of the oötype, instead of being terminal, has now a lateral or rather a ventral position, so that the spine which occupies this portion, instead of being terminal, is now lateral. It is noteworthy that these lateral-spined eggs are thicker, owing to the excess of material present, and not uncommonly have a curved anterior border, due to a projection of the anterior end into the anterior opening of the oötype.
As these eggs are being laid by females in the portal vein they are carried back to the liver by the blood stream. The liver is one of the commonest sites for these eggs; also terminal-spined eggs may be found here for the same reason.