Selected monographs on dermatology / P.G. Unna [and others].

Date:
1893
Catalogue details

Licence: Public Domain Mark

Credit: Selected monographs on dermatology / P.G. Unna [and others]. Source: Wellcome Collection.

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    pathological circumstances which attack profoundly the whole constitution. This is easily understood when we con- sider that the normal constriction of the vessels is consider- able, and that the vaso-motors must be intensely stimulated in order to produce universal visible pallor and sensible cold. We may, therefore, safely ignore a paralysis of the dilators in the production of general cutaneous anaemia. In contradistinction to the general causes which produce universal anaemia we have to look for local causes for the production of localized anaemias. These can be more easily classified into those due to primary vascular constriction, and those in which the narrowing of the calibre of the vessels is secondary to a diminished blood supply. We make frequent use of the latter principle, and increase the vascular tone ; for instance, when we raise the extremities to an unwontedly high position, for the purpose of quieting the circulation or lessening pain. Ligaturing an artery causes immediate contraction of its trunk and branches on the distal side of the ligature, and in the case of a vein, on the proximal side as far as the first collateral branch; and the contraction accurately represents the pressure of the column of blood which has been removed. Perceptible paleness is rarely produced in these cases—only temporarily, indeed, even after ligaturing a large artery such as the femoral, for the collateral circulation is soon established. The anastomoses of the superficial and deep cutaneous net- works of vessels are far too extensive for the ligaturing of a few of them to make any impression on an area of skin. A partial emptying of the vessels through external pres- sure is another cause of local anaemia. The compression may be from without the body, as from the finger, articles of dress, bandages, &c, or from within, from tissue masses, &c, beneath the cutis. While arterial ligature generates localized anaemia with difficulty, pressure on the arterioles and capil- laries produces it with ease, but only so long as the pressure lasts ; on its removal, the anaemia is replaced by a relaxive hyperaamia—the vessels having become paretic through the pressure. In this manner, too, act tumours of recent date which stretch the skin, collections of fluid and foreign bodies, as well as rapidly growing abscesses, temporary oedemas,
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