On the Composition of the Blood. 
207 
vortcd into cliylo have been fully set forth in the lecture before 
referred to, and this being the case it will be only necessary to 
direct the reader's attention to the explanations therein given. 
In comparing the quantity of blood with the entire weight of 
an animal, it will be found difficult to arrive at the exact propor- 
tion they relatively hold to each other, but it is sufficient for 
our present purpose to state that the amount of blood is usually 
estimated at from one-fourth to one-fifth of the entire weight of 
the body. 
If we were to attempt to give a popular definition of the blood, 
it might be described as a fluid which circulates through the 
heart, arteries, and veins, carrying with it the materials which 
are indispensably necessary for the maintenance of life, heat, 
nutrition, renovation, and secretion, building up the organism of the 
young animal and supporting that of the adult and aged. To effect 
the passage of the blood from one part of the system to another, 
various organs are employed and several forces brought into opera- 
tion. The chief organ for this purpose is the heart, which may be 
regarded as a central pump, having in connexion with it two sets 
of vessels — the arteries and veins : the former of these being trans- 
mitting, and the latter returning conduits. Besides these vessels 
there are intermediately placed between them, as it were, another 
set, called, from their small size, capillaries, to which we shall 
have occasion, hereafter, more particularly to allude. 
It is well known that in all the higher orders of vertebrate ani- 
mals the blood, as it appears to the unassisted vision, when drawn 
from its vessels, is red in colour. This redness, however, does not 
depend on any inherent colour in the fluid itself, but is due to 
an innumerable number of red corpuscles or cells which are 
floating within it. If, then, these bodies are removed from the 
blood, the true liquor sanguinis which remains behind will be 
found to be of a pale straw colour, resembling in this respect the 
blood of the invertebrate class of animals. 
It can be readily imagined that a fluid, which nature employs 
for such multitudinous purposes in the animal economy, is likely 
to be very complex in its elements, and such indeed is the case. 
To analyze these, even imperfectly, it is necessary, as a general 
rule, that the blood be first removed from its vessels. On this 
being done, it will be found shortly afterwards that a remarkable 
change takes place in it, and that it is now no longer fluid but has 
assumed a solid form. This phenomenon is among the most in- 
teresting which belong to the blood, and clearly indicates that the 
fluid possesses an inherent capability of conversion into organic 
structure. From the time of Hunter down to the present period, 
the correct explanation of the phenomenon of coagulation — 
clotting — has occupied the attention of our ablest chemists and 
