This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
When Blood Is Examined Microscopically it is seen to contain two kinds of corpuscles, the coloured kind already alluded to, and a less numerous set of white corpuscles.
The coloured or red corpuscles are, properly speaking, deep orange, as may be seen by streaking blood on a white surface. They are disc-shaped bodies, about 1/3200 inch diameter in man, circular, and flat or somewhat concave on each side. They are clear, and in mammals are destitute of nucleus; but this is a mammalian peculiarity, for in all other vertebrate animals they have a nucleus and are ovaL In the camel tribe they are likewise oval, but are destitute of nucleus as in other mammals, In different vertebrate animals the red corpuscles differ greatly in size, as indeed do other textural elements. Their size is dependent more on the affinities of the animal than on its bulk. In ruminants generally they are small; and in the smallest ruminant, the musk deer, their diameter is only of an inch. In birds they are smaller than in reptiles; and those of greatest size are found in the amphibia, the largest known being those of the protein, which are 1/400 of an inch in length.
Red corpuscles contain a firm framework or stroma, besides their coloured contents; but it is difficult to believe that they have any envelope, when one sees the great power of elongation which they have in threading their way through narrow passages, and the changes of shape which they undergo in various circumstances outside the body, without exposure of a membrane. They may often be seen to become indented round the edges; and the processes between the indentations may grow to a length which seems inconsistent with the supposition that they are firmer toward the surface, than within.
In blood which has a tendency to " buff," the red corpuscles are seen under the microscope to arrange themselves in columns like rows of coin, their cohesive attraction one to another being increased, or that between them and the liquor sanguinis being diminished. The main function of the red corpuscles is, as we shall find, to carry oxygen.