SERUM OF BLOOD. 79 



tracts. This is affected by many circumstances : generally, 

 the faster the coagulation the less is the amount of con- 

 traction ; and, therefore, when blood coagulates quickly, it 

 will appear to contain a small proportion of serum. Hence, 

 the serum always appears deficient in blood drawn slowly into 

 a shallow vessel, abundant in inflammatory blood drawn into 

 a tall vessel. In all cases, too, it should be remembered, that, 

 since the contraction of the clot may continue for thirty-six 

 or more hours, the quantity of serum in the blood cannot 

 be even roughly estimated till this period has elapsed. 



The serum is an alkaline, slimy or viscid, yellowish fluid, 

 often presenting a slight greenish, or greyish hue, and with 

 a specific gravity of from 1025 to 1030. It is composed of 

 a mixture of various substances dissolved in about nine 

 times their weight of water. It contains, indeed, the 

 greater part of all the substances enumerated as existing 

 in the blood, with the exception of the fibrin and the red 

 corpuscles. Its principal constituent is albumen, of which 

 it contains about 8 per cent., and the coagulation of which, 

 when heated, converts nearly the whole of the serum into 

 a solid mass. The liquid which remains uncoagulated, 

 and which is often enclosed in little cavities in the coagu- 

 lated serum, is called serosity : it contains, dissolved in 

 water, fatty, extractive, and saline matters. 



Variations in the principal Constituents of the Liquor Sanguinis. 



The water of the blood is subject to hourly variations in its 

 quantity, according to the period since the taking of food, 

 the amount of bodily exercise, the state of the atmosphere, 

 and all the other events that may affect either the ingestion 

 or the excretion of fluids. According to these conditions, 

 it may vary from 700 to 790 parts in the thousand. Yet 

 uniformity is on the whole maintained; because nearly 

 all those things which tend to lower the proportion of water 

 in the blood, such as active exercise, or the addition of 

 saline and other solid matter, excite thirst ; while, on the 



