These terms designate the condition in which the blood-vessels, and in particular the capillaries, are more or less empty of blood, and the part is correspondingly pale.
The vessels, and especially the capillaries, may be directly emptied by pressure from without. We have an artificial anaemia produced in this way by Esmarch's elastic bandage. A tumour or an abscess may by pressure empty the vessels, and, if long continued, this may lead to necrosis of the structure concerned. In most cases, however, anaemia is due to obstruction of arteries, but such obstruction rarety produces ischaemia unless it be comparatively sudden and tolerably complete.
Spasm of arteries, that is to say, violent contraction of their middle coat, may cause such obstruction as to produce an extreme anaemia. The application of cold to the skin causes contraction of the cutaneous arteries - ether spray causes a striking anaemia by this means. The rigor at the beginning of many fevers, accompanied as it is by paleness and coldness of the surface, is due to spasm of the cutaneous arteries. Some neuralgias are associated with spasm of arteries. (Du Bois-Key-mond.) In some persons the vaso-motor system is peculiarly sensitive, and slight causes are sufficient to induce spasm of the arteries and a local anaemia.
A series of phenomena produced by prolonged spasm of arteries is grouped under the name of Raynaud's disease, from the writer who first gave a full description of these phenomena. In predisposed persons an ordinary exposure to cold, as in washing, will induce such a spasm as to render the fingers bloodless and anaesthetic; they are said to be "dead" (local syncope). In more severe cases the skin becomes dark blue and various eruptions may form (local asphyxia). In very severe cases there may be actual necrosis of the ends of the fingers and toes. This form of disease is symmetrical.
A sudden obstruction of an artery by Embolism is a frequent cause of Ischaemia, but this frequently gives place to a passive hyperaemia. (See under Embolism.) The ligature of an artery or a sudden compression will also lead to local anaemia.
Disease of the walls of arteries is not a frequent cause of ischaemia, as obstruction from such conditions as atheroma is frequently very partial, and even when considerable is of slow production. Hence such conditions more frequently lead to passive hyperemia in the way already mentioned.
In the anaemic part the capillaries are imperfectly filled, and the blood current is slow. The part is consequently pale, reduced in temperature, flaccid. Its nutrition is diminished, and its elements are prone to undergo atrophy and degeneration, or even, as we have already seen, necrosis. The function will be interfered with if the nutrition is depreciated, and if the anaemia affect an important organ, the results may be serious. Thus, obstruction of the coronary arteries may cause death by paralysis of the heart. Lastly, a local anaemia may produce a hyperaemia elsewhere - a collateral hyperaemia.
Recklinghausen's Handbuch, p. 35; Virchow, Handbuch d. spec. Path, und Therapie, vol. i.; Raynaud, De l'asphyxie locale et de la gangrene symetrique, 1862; and Arch. gen. de med., xxiii., 1874; Du Bois-Reymond, Arch, f. Anat. und Physiol., 1860.