Passive HypercEmia. 117 



it to escape more or less freely according to the pressure, and may 

 in fact, rupture. However, even if the blood be not forced into the 

 area from the arteries, exudation of the fluid part of the blood 

 through the distended capillary walls may take place in ordinary 

 atonic hypostatic congestion, and there may even occur a diapcdesis 

 of red blood cells. The endothelium and vessel walls remain intact 

 only while they are being continually bathed with moving blood ; and 

 as soon as the circulation stops it may be accepted that a disturb- 

 ance of the endothelial nutrition exists, these cells then shrinking 

 and causing comparatively large open interstices, through which 

 exudation takes place. For these reasons passive hyperemia has 

 frequently associated \/ith it a congestion-transudation and haemor- 

 rhage. The latter may be sufficiently severe to make the whole 

 tissue dark red in color, infiltrated and completely occupied by 

 blood (haemorrhagic stasis, h?emorrhagic infarction). 



The pressure occasioned by the distended vessels and the fluid 

 exudate, together with the impediment to circulation, may be fol- 

 lowed by cessation of function of the aft'ected tissues and finally 

 by their death. The longer the blood remains in the capillaries, 

 slowly giving oft' its oxygen and capable of but little or no renewal, 

 the darker it becomes ; the tissues in venous hyperemia arf there- 

 fore of a dark bluish red (cyanotic) hue. Such a tir.t (diffuse 

 cyanosis, black erythema) may be encountered (in hogs) extending 

 over practically the whole surface of the body in case of general 

 passive hyperaemia. The lack of oxygen and the excess of carbon 

 dioxide in the venous blood are partly responsible for the functional 

 disturbances arising from passive hypersemia of the various organs ; 

 these conditions act by stimulating the medulla, and occasion 

 dyspnoea, dizziness, convulsions, disturbances of consciousness, 

 muscular weakness and fatigue. 



The superficial external parts of the body, when passively con- 

 gested, become cool because of the impeded circulation of blood 

 through them, because the blood is cooler than normal and the heat 

 dissipation not diminished. 



In contrast to wdiat has been said above, a mild but persistent 

 passive hypersemia is apt to cause not only a swelling of the organ 

 affected (kidneys, lungs, liver, spleen) because of the permanent 

 engorgement and distention of the capillaries, but in addition may 

 induce an increase in size and induration from the production of 

 an excess of connective tissue in the part (Cf. hypertrophy). 



