REMARKS ON COMPOSITION OF THE BLOOD. ?27 
We shall now proceed to a consideration of the red cells 
-—the colouring matter of the blood. 
The Red Cells. —It has already been said that the redness 
of the fluid is entirely due to certain cells which are floating 
within it, commonly designated the red particles. These 
bodies exist in such vast numbers, that many hundreds may 
be said to be present in every drop of blood, and it has been 
estimated that, on the whole, they constitute no less than an 
eighth part of the entire quantity of the circulating fluid. 
The discovery of the red cells is said to have been made by 
Malpighi, a celebrated Italian anatomist, who flourished in 
the latter part of the seventeeth century. Since his time, 
they have excited the liveliest attention on the part of all in¬ 
vestigators of the blood, which has led to a more complete 
knowledge of their structure, as well as of their uses in the 
animal economy, than had previously existed. 
The aid of the microscope is indispensable even for obtaining 
cognisance of their presence, and our more extended knowledge 
of these cells is, in a great measure, due to the improvements 
which have of late years been made in the defining powers of 
this instrument. In man, and in most of the mammalia, the 
red cells are circular in shape, but in birds, reptiles, and fishes, 
they are oval. The exceptions to the circular shape in mam¬ 
mals are met with in the camel, the alpaca, and their allied 
species, in which the cells have, as in birds, an oval form. It 
is not to be inferred because the red cells are round, that they 
are therefore globular-shaped bodies; for, having flattened 
sides, they rather resemble the form of an ordinary coin. 
Correctly speaking, even their sides are not flat, but slightly 
concave, so that the cells may be described as bi-concave 
circular discs. This is their more general and, it may be said, 
perfect shape, but as they readily imbibe fluid through their 
pellucid and colourless walls, so, by an addition to their 
contents, will their sides become first flat, and afterwards 
convex, according to the amount which is absorbed. 
Their size is likewise liable to great variation in different 
animals, and even in the same animal it is not uniform. In 
man their diameter varies from the l-3000th to 1 -4000th of 
an inch, and their thickness is about 1-10000th of an inch. 
According to the measurements of Mr. Gulliver, given in an 
appendix to Gerber’s f Elements of General and Minute 
Anatomy,’ the average diameter of the red cells of the horse 
is l-4?06th of an inch; of the ox l-4267th; the sheep l-5300th; 
the pig l-4230th; and the dog l-3542d. In the goat and 
deer tribe, they are smaller than in the sheep, reaching their 
smallest known size in the Napu musk deer, in which their 
