PATHOLOGY.--In the foregoing description those extravasations of blood due to simple mechanical violence, as from flea-bite, and sudden increase of blood-pressure, as in the effort of coughing in whooping cough, also from the deleterious influence exerted upon the blood-vessels and blood by certain drugs, the specific fevers, Bright's disease, and the like, have been excluded. Only those have been considered where the effusion of blood seemed to occur spontaneously, and the symptoms to result from some peculiar but not understood morbid process. The hemorrhage is but a symptom; the process by which it is brought about depends upon some change in the blood or blood-vessels. We do not know what these subtle changes are. The blood of purpuric patients has been carefully examined, but, with the exception above mentioned, no definite changes have been discovered. Immermann[11] found during the first stage of the disease the blood-corpuscles perfectly normal in appearance, the white corpuscles subsequently slightly exceeding the red in number--a simple result of copious hemorrhage. No stated chemical changes in the blood are known in purpura, nor is it known how the blood escapes from the vessels. It undoubtedly escapes through alterations in the vascular wall, but it is also true that red blood-corpuscles, as well as the pale ones, may find their way in considerable numbers through the unruptured wall of the vessels, per diapedesin, as was first suggested by Velpeau, but definitely determined by Stricker. The causes of this migration are obscure. Immermann[12] asserts that a fatty degeneration of the vascular tissues and of the muscles takes place. This, however, is manifestly a result of the loss of blood, and not its cause. Dr. Wilson Fox[13] found extensive albuminoid disease of the muscles and capillaries of the skin; but the albuminoid degeneration involved several organs of a patient with syphilis, and the purpura was certainly secondary to the morbid conditions. Rigal and Cornil[14] think that the hemorrhages are a result either of sympathetic irritation or of diminished action of the vaso-motor centre. It is indeed altogether likely that the cause will ultimately be found to reside in the vaso-motor system. [Footnote 11: _Ziemssen's Cyclop._, xvii. p. 258.] [Footnote 12: _Loc. cit._] [Footnote 13: _Brit. and Foreign Med.-Chir. Review_, Oct., 1865.] [Footnote 14: _L'Union Méd._, 5, 6, 7, 1880.]