SERUM OF BLOOD. 73 



the less is the amount of contraction ; and, therefore, when 

 blood coagulates quickly, it will appear to contain a small 

 proportion of serum. Hence, the serum always appears de- 

 ficient 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 grayish 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 prin- 

 cipal 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 inclosed in 

 little cavities in the coagulated serum, is called serosity; it con- 

 tains, 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 other hand, the addition of an ex- 

 cess of water to the blood is quickly followed by its more 

 copious excretion in sweat and urine. And these means for 

 adjusting the proportion of the water find their purpose in 

 maintaining certain important physical conditions in the 

 blood ; such as its proper viscidity, and the degree of its ad- 

 hesion to the vessels through which it ought to flow with the 

 least possible resistance from friction. On this also depends, 

 in great measure, the activity of absorption by the bloodves- 

 sels, into which no fluids will quickly penetrate, but such as 

 are of less density than the blood. Again, the quantity of 

 water in the blood determines chiefly its volume, and thereby 



