To their more exact study we must, however, make some preface in the way of general remarks concerning a phenomenon everywhere easily recognized, but as yet incompletely understood. This phenomenon has reference to an undue supply of blood to a part, and is commonly known under two terms which are practically synonymous, namely, _congestion_ and _hyperemia_. To begin with these, then, we must note, first of all, that congestion and hyperemia may be-- A. _Active_; and B. _Passive_. They may also be spoken of as-- 1. _Acute_; and 2. _Chronic_. Considering first the two latter distinctions, it will be found that the acute hyperemias are met with most often in consequence of sharp mechanical disturbances. The chronic hyperemias, on the contrary, are conditions which in many individuals are more or less permanent. Note accurately here the proper significance of certain terms. Hyperemia means, in effect, an oversupply of blood to the given part; the term should have only a local significance. When the entire body seems to be too well supplied with blood, the condition is known as _plethora_, the counterpart of which term is usually _anemia_. The direct counterpart of the term _hyperemia_ should perhaps be _ischemia_, meaning a perverted blood supply in reduced amount. With plethora and anemia as terms implying general conditions, with hyperemia and ischemia implying local conditions, there should be little room for confusion in phraseology. The active form of hyperemia used to be called “fluxion,” a term now rarely used. _Active hyperemia_ means an increased supply of _arterial_ blood. In _passive hyperemia_ the oversupply is rather of _venous_ blood. In the former case the condition seems due to overactivity of the heart, with such local tissue changes as permit it to occur. In passive hyperemia the blood current is slower--there is a tendency toward, and sometimes an actual, stagnation; all of which is usually due to obstruction of the return of blood to the heart. The conditions permitting these two results may be widely variant.