208 
THOMAS WALLET. 
the animal heat. The actual agent by which their oxygen-convey¬ 
ing function is performed is iron, and it is upon this element also 
that the cells are mainly dependent for their red color. Defi¬ 
ciency of iron means deficiency in oxygen, and the blood assumes 
a dark color—becomes venous, instead of being a bright red or 
scarlet color, as in arterial blood. 
The existence of a third corpuscle is now recognized by many 
physiologists, but to the character and functions of this I need 
not allude in detail, further than to say that it is believed to be 
the originator of the other cells; hence it was called by Hayem a 
hcematoblast, or blood-former. 
That the blood is possessed of vitality is shown by its power, 
on withdrawal from the vessels, of undergoing coagulation—a 
process brought about not by the existence, as was once supposed, 
of fibrin as fibrin in the blood, but by the action of a ferment 
upon two substances pre-existent in that fluid, known as fibrinogen 
and fibrinojplastin. 
The second problem we have to consider is the means by 
which the vitality of the blood is maintained. 
I have already said that the source of the blood is the alimen¬ 
tary matter we ingest or take into our stomach and bowels in the 
ordinary processes of eating and drinking, and again I repeat that 
in order for the preservation of the equilibrium in the different 
constituents of the blood, the ingested matter must contain all 
the elements, and that too in due proportion, necessary for their 
supply. In other words, the food must contain a due proportion 
of flesh-forming matter, of heat producers, of alkaline salts (potash 
and soda), iron, phosphorus or phosphoric acid, magnesia and 
lime; with a few other elements of a less important character, 
such as sulphur. Before considering the effects of improper ali¬ 
mentation in the direction of inducing diseased processes, it will 
be useful to glance briefly at the effects of adverse agencies on the 
blood itself. Looking at the blood in the light of its vast impor¬ 
tance in the animal economy, it would seem at first sight that it 
ought to be a stable tissue. Nothing, however, is further from 
the truth than is such a supposition, as even quantitative deficien¬ 
cies or excesses in its elements are sufficient to induce grave 
