Hypcrcctuia ami Anccmia. iii 



affections which interfere with the aspiration factor of venous cir- 

 culation and h\- thromhosis of the veins ; but it should also be kept 

 in mind that any abnormal compression of these thin-walled veins 

 must interfere with the return flow of blood. The nearer the 

 heart such compression occurs the greater the harm to the gen- 

 eral body. Compression of the two venae cavre, which from the 

 compressibility of these vessels is quite possible from pleural effu- 

 sions, induce a reduction in the amount of blood entering the 

 cardiac chambers and venous congestion throughout the economy 

 (cf. hyperconiia). 



The lymph supply and drainage are in close relation 

 with the abnormalities of the blood vascular system and blood 

 circulation ; imperfections in the lymphatic circulation give rise to 

 abnormal accumulations of fluid in the lymph vessels and serous 

 cavities and occasion a wide group of organic lesions. 



Local Variations in Amount of Blood. Hyperaemia and Anaemia. 



The amount of blood in the organs of even healthy individuals 

 is always subject to change and variation. The vessels are elastic 

 tubes which are expanded by greater internal blood pressure and 

 adjust themselves as their contents diminish. This is not merely 

 a passive accommodation, however ; they are capable of active 

 contraction and dilatation through the power of the smooth muscle 

 in their structure, such alterations of lumen being primarily under 

 control of nervous influences. The total quantity of blood in 

 the body, similarly subject to continual variation, is never so great 

 as to completely fill all the vessels were they fully dilated ; there 

 exists, however, a moderate fullness, varying here and there in 

 the organs as the blood-current meets resistance to its progression 

 or as the blood is drawn into special organs from dilatation of 

 this or that vascular area. The quantity of blood in the organs, 

 and the hsemic distribution in the body generally, physiologically 

 accommodates itself to the existing demands. More blood enters 

 an organ when functionating; the amount traversing it dimin- 

 ishes during periods of rest. The nervous stimulus which causes 

 an organ to actively functionate, at the same induces dila- 

 tation of its vessels, and in consequence a rich supply of blood is 

 afforded. This is especially recognized in muscle. Ranke 

 determined the blood content of the general musculature of rab- 

 bits at rest to be 36.6 per cent, of the total muscle bulk, and noted 

 this proportion nearly doubled (66 per cent.) when the muscles 



