Oil the Composition of the Blood. 
213 
their size in ruminants seems to prevail, the cells becoming 
smaller with the (llniinution of the size of the animal. This, 
however, is so far from being the case among the mammalia in 
general, that it is ordinarily stated that the bulk of an animal has 
little to do with the size of the red cells of its blood. It would 
aj)j)ear, however, from recent investigations of Professor Quekett 
that the calibre of the capillary vessels of each individual animal 
is to be taken as a more correct standard of the size of the red 
cells than anything else — the two rising or falling together. 
A great deal of discussion has taken place in former times, as 
to whether these cells were, or were not, nucleated in mammals ; 
different observers of equal eminence maintaining opinions the 
very opposite of each other. The matter is one of some im- 
portance, as elucidating the probable means of their reproduction. 
Like every other part of the organism, they undergo changes which 
result in their ultimate dissolution or breaking up. If then they 
were nucleated it is evident that they would follow the same law 
as all other cells of this class in their reproduction, namehjy the 
setting at liberty of their nuclei by disintegration, which would 
then develope into new cells. The opinion that they possess 
a nucleus has doubtless had its origin in the circumstance that, 
when viewed as transparent objects, their bi-concave form gives 
them an appearance of having dark centres, from the refraction 
to which the rays of light are exposed. That this is the true cause 
of the phenomenon, is proved by placing these bodies in a fluid 
less dense than that which they contain, when, by their imbibition 
of a portion of this, the dark spot disappears ; they being thus 
changed from bi-concave to flat-sided or even double-convex 
discs. This procedure would, on the supposition that they were 
nucleated, tend, however, to bring the nucleus more into vision. 
The converse also is equally true, namely, the rendering the dark 
spot more distinct by emptying them of some of their contents, 
which is accomplished by placing them in a fluid of greater 
density than that which is located in their interior. 
It is a singular fact that, when viewed in connexion with the 
blood-cells of other creatures, those of mammals should be ?;rtnu- 
cleated. In birds, reptiles, and the amphibia, the red cells possess 
a nucleus ; they are also very much larger than in the mammalia, 
a circumstance which affords many advantages for the study of 
their structure, &c., in these creatures. 
The proportionate quantity of the red cells to the other con- 
stituents of the blood has already been said to be as much as 
an eighth part. The quantity, however, is liable to much 
variation, depending on certain conditional states of the system. 
In animals of robust health it is always large, as also in those 
that are well fed and which undergo a fair amount of exertion 
