Modern surgery, general and operative . penings enlarge (page 93). Fluidelements rarely leave the blood-vessels during active hyperemia, but theyoccasionally do. The wheals of urticaria are thus formed (Warren). Activehvperemiais often the first stage of an inflammation, but it is not of necessityfollowed by other inflammatory changes, and it can be caused by nerve sectionor nerve stimulation. The duration of active hyperemia is variable. If the irritation has beenbrief, the hyperemia is very transitory. In some cases dilatation with acceleratedcirculation is scarcely more than momentary, givi
Modern surgery, general and operative . penings enlarge (page 93). Fluidelements rarely leave the blood-vessels during active hyperemia, but theyoccasionally do. The wheals of urticaria are thus formed (Warren). Activehvperemiais often the first stage of an inflammation, but it is not of necessityfollowed by other inflammatory changes, and it can be caused by nerve sectionor nerve stimulation. The duration of active hyperemia is variable. If the irritation has beenbrief, the hyperemia is very transitory. In some cases dilatation with acceleratedcirculation is scarcely more than momentary, giving way almost immediately todilatation with retardation. If the irritation is prolonged, hyperemia may lastsome time before giving way to retardation. In the web of a frogs foot, if anirritant is applied, hyperemia lasts from one-half hour to two hours before it isreplaced bv retardation. Clinical Signs of Active Hyperemia.—A hyperemic part, if on or near thesurface, is red in color, imparts a sense of heat to the examining hand, the. Fig. 46.—Retardation of blood andmigration of white corpuscles in in-flammation. Resume of the Vascular Changes of Inflammation 91 color quickly disappearing on pressure and quickly returning when pressureis released. In a congested part the temperature is diminished, the surface ispurple, the congested veins are visible, there are edema and a sensation ofcoldness and numbness. When congestion is purely local the lividity dis-appears quickly when pressure is appHed and returns quickly when pressureis removed. When due to disease of the heart or lungs, lividity disappears andreturns slowly. When a local congestion is about to give way to gangrenethe lividity disappears very slowly on pressure and crawls back very slowlywhen pressure is released. Retardation.—After active hyperemia has existed for a variable time theblood-current begins to lessen in velocity, until it becomes more tardy thanin health. This is known as retardation of the circul
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