Text book of veterinary medicine, Volume 2 (of 5)

by James Law

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.

The divisions of the left bundle anastomose so freely with each other in the mesentery and immediately above the intestine that the blocking of any one branch cannot entirely arrest the circulation in the corresponding part of the intestine. It may however produce a partial local stagnation in the vessels of a short loop of intestine, resulting in œdematous infiltration and thickening with resulting induration and stricture of the gut. Chronic and permanent lesions are produced by such blocking, but only rarely acutely fatal ones. Acute and fatal congestive lesions of the small intestine from verminous embolism, occur only when several adjacent divisions of the artery are blocked at once, and this is a rare occurrence.

The right bundle of branches furnishes the only two arteries which are supplied to the cæcum and the only artery furnished to the first half of the double colon. The ileo-cæcal branch is less involved, first, because being less dependent and smaller, it is less likely to receive an embolus, and, second, because any lack of blood supply is counterbalanced by the free anastomosis with the last iliac division of the left bundle. When the embolus blocks the undivided trunk of the right bundle this same principle comes into play, the free supply of blood from the posterior branch of the left bundle supplying blood through its anastomosis with the iliac and cæcal branches of the right.

But when the emboli are lower down, in the cæcal branches of the right bundle, or in these and the colic branch, arrest of the circulation in the intestinal walls ensues, followed by paresis, passive congestion and hemorrhage. The cæcum and double colon thus become the seats of the grave and fatal lesions of verminous embolism.

The resulting lesions are to be variously accounted for. The stagnation of blood in the vessels below the embolus, determines a speedy exhaustion of its oxygen and increase of its carbon dioxide, so that it is rendered unfit to maintain the normal nutrition and functions of the part, and the capillary and intestinal walls are alike struck with atony or paresis. The blood filters into the stagnant vessels slowly from adjacent anastomosing trunks, and the liquor sauguinis exudes into the substance of the tissues and lumen of the intestine, leaving behind the greater part of the blood globules so that the stagnant blood is rendered more and more abnormal in composition. The walls of the capillaries soon lose their cohesion as well as their contractility, and giving way at different points, allow the escape of blood into the tissues, bowels and peritoneal cavity. It has been further claimed that the emboli already infected and in process of degeneration communicate this to the walls of the vessels and to the stagnant blood, hastening the process of degeneration and rupture.

Another series of circulatory disorders are liable to take place. The blocking of the vessels of the right bundle, tends to increase the blood pressure in the left bundle and the anterior one, and thus to determine congestions, paresis and inflammations in the small intestines, the second division of the double colon and the floating colon. The resulting inflammation and increased vascular tension may lead indirectly to implications of the brain and lung.

Extravasations so extensive as to appear like blood clots may be present between the layers of the mesentery or in the mucosa and submucosa, and blood, liquid or coagulated, may have accumulated in the abdominal cavity. Blood effusion into the intestine gives a dark red coloration to the contents which are further mixed with distinct clots.