The irritation of the intima from whatever cause determines here as elsewhere exudation, and coagulation, and the inflamed walls losing their tone yield more and more readily to the internal tension. Sometimes the coagulum lines the aneurism or vessel all round, leaving a narrow central passage through which the blood still flows; in other cases the clot extends into the adjacent smaller vessels, completely blocking them and disturbing circulation and innervation in the parts which they supply. As a rule the parasites are found in galleries hollowed out in the clot, and heads or tails may be seen to project into the circulating blood. Sometimes they are found imbedded in the arterial coat, or in an adjacent small abscess. The formation of aneurisms in the other arterial trunks may follow the same method. =Embolisms.= These come very naturally from the formation of thrombi in the various arteries. The coagulum determined by the presence of the worms, tends to undergo retrogressive changes notably fatty degeneration, to which germs brought on the worms or in their alimentary canals contribute. This together with the movements of the parasites tends to break up the mass, and minute portions are washed on into the different smaller vessels. Soon these reach divisions which are too small to admit them, which are accordingly occluded and the circulation through them abolished. The presence of microbes as well as fibrine contributes to cause further coagulation, more absolute embolism and arrest of the circulation. It is further alleged that the sexual instinct in the summer months (May to August) leads the worms to leave the aneurisms, to pass through the smaller divisions to the cæcum or colon where alone full sexual evolution is possible. In these migrations they cause the thrombosis of the smaller trunks and determine the verminous congestions of the bowels which are especially common in these months. =Disturbances of the Intestinal Circulation.= As these usually occur in the lines of distribution of the anterior mesenteric artery a knowledge of its divisions and their destination and anastomosis, is essential to an intelligent understanding of the pathogenesis and lesions. As first pointed out by Lecoq the anterior mesenteric artery is divided into three primary bundles: (a) a _left_ of 15 to 20 trunks which are destined to the small intestine; (b) a _right_ which gives off cæcal branches, one to the double colon, and one to the ilium to anastomose with the last trunk of the left bundle; and (c) an _anterior_ which gives one branch to the second division of the double colon and anastomosis with the colic branch of the right bundle at the pelvic flexure; and a second branch to the floating colon to anastomose with the posterior mesenteric artery.