208 
0/i the Cumpositiu/i of the Blood. 
physiologists ; and perhaps it is not too much to say that, not- 
withstandin<^ all the light which has been shed upon it, some 
darkness still enshrouds the solution of the problem. 
The time which elapses before the blood becomes solid will 
vary considerably, depending on many adventitious circum- 
stances. The coagulation is usually effected in ten or fifteen 
minutes, but in some instances jnany hours and even days will 
pass before it is completed. Before alluding more particularly to 
either the clotting of the blood or the variations in tlie time 
required for its accomplishment, it will be necessary to describe 
the principal component parts of the fluid. 
On setting aside the coagulated mass and keeping it at rest for a 
short period, a transparent fluid is found to exude from it, which 
can easily be decanted off. This is the serum, or so-called 
watery part of the blood. 
The Serum. — Under all ordinary circumstances this consti- 
tuent of the blood remains in a fluid condition, while the quantity 
which is exuded will be in proportion to the time the coagulum 
remains at rest, until the expiration of about thirty hours ; after 
which, however, but little more will be expelled naturally, although 
a still greater amount can be obtained by drying the clot. It is 
therefore evident that by the act of coagulation the serum is me- 
chanically enclosed in the solidified mass, and that subse([uently, 
by this undergoing a certain degree of condensation, the greater 
part of it is squeezed out as water is expelled from a sponge by 
the application of pressure. 
Thus obtained, the serum is found to be a viscid fluid of a 
yellowish colour and having an alkaline reaction. Its specific 
gravity varies from alx)ut 1*025 to 1*050. It is a very important 
element of the blood, containing not only the watery and saline 
materials of that fluid, but also the albuminous matters in a state 
of free solution. In short, it may be said to include the principal 
portion of all the constituents of the blood with the exception of 
the fibrine, the hcematine, and the globuline. The proportion 
which its several parts bear to each other will materially depend on 
certain conditional circumstances, such as the kind of food on 
which an animal is fed, the state of its health, the uses to which it 
is put, the temperature to which it is exposed, 6cc. Notwithstand- 
ing these disturbing causes, if the vital forces are still active, the 
balance is fairly maintained. Thus, speaking in general terms, 
every 1000 parts of serum contain about 780 of water ; and 
although this quantity, even in health, is subject to variation, 
and may sometimes rise to 790 or sink to 700, the first-named 
quantity is nevertheless present as a rule. Any diminution in the 
amount of water is quickly compensated for by the thirst which it 
creates, while any excess will be as rapidly removed by the skin 
