When the embolus reaches a point beyond which it cannot pass, the resulting disturbance depends essentially, as shown by Cohnheim, upon the presence or absence of arterial anastomoses beyond the place of obstruction. He gives the name terminal arteries to those which have no anastomosing arterial branches. These are met with in the spleen, kidneys, lungs, brain, and retina. If the obstructed artery is not terminal, the embolus may produce no further disturbance, the collateral supply of blood through the anastomoses sufficing for the nutrition and function of the part. If, however, the vessel is a terminal artery, and the embolus is completely obstructing, the supply of arterial blood must be wholly cut off from the region beyond the seat of obstruction.
If the embolus does not completely obstruct at once, it soon becomes sufficiently large for this result to ensue in consequence of a secondary coagulation. The rider assumes legs extending into the arterial branches beyond the place of obstruction, and a body which extends backward in the course of the circulation to the nearest branch. The result of the total obstruction of the vessel is to cut off the admission of arterial blood, producing a local anæmia. The contraction of the elastic tissues of the part propels toward the capillaries a certain quantity of the blood in the vessels beyond the point of obstruction, till this force becomes neutralized by the blood-pressure in the vessels surrounding the obstructed region. The anæmic part may subsequently become engorged with blood; it may die, a region of anæmic necrosis resulting, or the dead portion may become softened.
The engorgement of the obstructed territory has received the name of hemorrhagic infarction. A solid, wedge-shaped mass of a reddish-brown color is present, whose shape is due to the arborescent branching of the terminal arteries. According to Cohnheim, the engorgement of the region with blood takes place from venous regurgitation into the obstructed part, till the intravenous pressure is overcome by the resistance of the tissues in the region affected. The capillaries and larger vessels thus become distended, and an escape of liquid and solid constituents of the blood takes place. If the veins are provided with valves, or the venous regurgitant current is opposed by gravity, the hemorrhagic infarction is prevented or greatly impeded.
Litten,22 on the contrary, who has furnished a recent contribution to this subject, claims that the hemorrhagic results of embolism are not accomplished through venous regurgitation, unless increased venous tension is produced by coughing, vomiting, and like efforts. His experiments lead him to maintain that arterial blood from surrounding tissues is supplied to the obstructed region through the anastomosing capillaries. The force is not sufficient to drive the blood through the capillaries into the veins beyond, but an accumulation takes place in the capillaries, which become dilated and distended. The escape of blood-corpuscles and serum then takes place, the more freely, as Weigert23 suggests, the larger and more numerous are the pre-existing spaces in the organ. Hence the infarction becomes the most characteristically developed in such organs as the lungs and spleen. Causes which obstruct the venous flow, as well as those which increase the arterial tension, promote the hemorrhagic infarction.
22 Untersuchungen über den hemorrhagischen Infarct., etc., Berlin, 1879.
23 Virchow's Archiv, 1878, lxxii. 250.
A necrosis of the part whose direct arterial supply is cut off takes place when the structure of the organ affected is such that the admission of arterial blood is wholly interfered with. This is the case in the heart and kidneys, and to a less extent in the spleen. The opportunity is presented for the diffusion of a fibrinogenous fluid, lymph or blood-serum, through the cells of the organ which contains the other essentials for coagulation, and the dead part presents the characteristics attributed by Weigert24 to death from clotting of the protoplasm, coagulative or ischæmic necrosis.
24 Ibid., 1880, lxxix. 87.
Embolism of the cerebral arteries produces softening of the brain, not a hemorrhagic infarction or a yellowish necrosis. Weigert attributes this result, on the one hand, to the absence in the brain of abundant cells from which are to be had the ferment and fibrino-plastic material necessary for coagulation, and, on the other, to the closure of the spaces into which blood might collect by the rapid swelling of the tissues from the exuded lymph.
The hemorrhagic results of embolism are also met with in obstruction of branches of the mesenteric artery, which is considered by Litten, at least from its function and in connection with its sluggish current, to correspond with a terminal artery.