212 
On the Coinjmition of the Blood. 
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 
(juantity 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 seventeenth 
century. Since his time they have excited the liveliest atten- 
tion on the part of all investigators 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 
cognizance of their presence, and our more extended knowledge 
of them 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 mammals arc 
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 
of this 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 j^i^j^ff to ^ly^jQ of an inch, 
and their thickness is about y ^,^,, of an inch. According 
to the measurements of Mr. Gulliver, given in an appendix to 
Gerber's ' Elements of General and Minute Anatomy,' the 
average diameter of the red cells of the horse is 47^^; ^" 
inch, of the ox 4^^? the sheep -^-^^fj, the pig 4:290' 
dog ^r}^2 - goat and deer tribe they are smaller than 
in the sheep, reaching their smallest known size in the Napu 
musk deer, in which their average diameter is said by Mr. 
Gulliver not to exceed the 757^25 of an inch. Gradation in 
