227. All the animal and saline matter held in solution in the serum being removed, the fluid that remains is water, the proportion of which in 1000 parts varies from 853, the maximum, to 779, the minimum.

228. The second constituent of the blood, the fibrin, is the most essential portion of it, being invariably present, whatever other constituent be absent. While circulating in the living vessel, fibrin is fluid and transparent; by the process of coagulation, it is converted into a solid and opaque substance of a yellowish white colour, consisting of stringy fibres, disposed in striæ, which occasionally form a complete net-work (fig. CXI.). These fibres are exceedingly elastic. In their general aspect and their chemical relations they bear a close resemblance to pure muscular fibre, that is, to muscular fibre deprived of its enveloping membrane and of its colouring matter, and they form the basis of muscle. According to M. le Canu, the proportion of the fibrin varies from seven parts in 1000, the maximum, to one part in 1000, the minimum, the medium of twenty experiments being four parts in 1000.

A portion of the fibrin of the blood, showing its fibrous
structure and the striated or net-like arrangement of its
fibres.

229. The third constituent of the blood, the matter upon which its red colour depends, though, as has been stated, entirely absent in certain classes of animals, and in all animals in some parts of their body, seems to be essential, at least to the organic organs, whenever they perform their functions with a high degree of perfection. Thus in the lowest class of vertebrated animals, the fish, while the principal part of its body receives only a colourless fluid, its organic organs, as the heart, the gills, the liver, are provided with red blood.

230. The red matter, wherever present, is invariably heavier than the fibrin, and consequently, during the process of coagulation, it gradually subsides to the lower surface, and is always found forming the bottom of the clot. Its proportion to the other constituents varies very remarkably, the maximum being 148, the minimum 68, and the medium 108, in 1000 parts of blood.

231. All observers are agreed that the red matter of the blood consists of minute particles, having a peculiar and definite structure; but in regard to the nature of that structure, there is considerable diversity of opinion, which is not wonderful, since the particles in question are so minute that they can be distinguished only by the microscope, and since, of all microscopical objects, they are perhaps the most difficult to examine, because, being soft and yielding, their figure is apt to change, and because there is reason to suppose that their substance is not uniform in its refractive power.

232. The earlier observers describe the red particles as being of a globular figure, and accordingly name them globules. They conceive that each globule consists of a central solid particle, enveloped in a transparent vesicle. Recently, Sir Everard Home and Mr. Bauer in this country, and MM. Prevost and Dumas on the continent, have revived this opinion, and describe the red particle as consisting of a central solid white corpuscle contained in an external envelop of a red colour. When the blood is observed with the microscope in a living animal, flowing in its vessels, only two substances can be distinguished, namely, a transparent fluid and the red corpuscles. MM. Prevost and Dumas contend that these two substances are the only component parts of the blood. When the blood coagulates, they conceive that the red envelop separates from the central white corpuscle; that these white corpuscles unite together; that the aggregates resulting from this combination are disposed in the form of filaments, which filaments constitute the fibrin, while the red matter at the bottom of the clot is nothing but the disintegrated envelops of the central particle. But this view is not the common one. In general, physiologists conceive the fibrin to be one constituent and the red particles to be another constituent of the blood. Mr. Lister, who has successfully laboured to improve the microscope, and who, together with his friend Dr. Hodgkin, have very carefully examined with their improved instrument the red particles, contend that the figure of these bodies is not globular, although they state that the instant the particles are removed from the living blood-vessels many things are capable of making them assume a globular appearance; such, for example, as the application of water. With a rapidity which, in spite of every precaution, the eye in vain attempts to follow, the particles change their real figure for a globular form on the application of the smallest quantity of pure water; while, if the water contain a solution of saline matter, little alteration is occasioned in the figure of the particles. According to these observers, the red particles are flattened cakes, having rounded and very slightly thickened margins (fig. CXII. 1). The thickness of the margin gives to both surfaces the appearance of a slight depression in the middle (fig. CXII. 1), so that the particles bear a close resemblance to a penny piece. There is no appearance of an external envelop. The circular and flattened cake is transparent; when seen singly it is nearly if not quite colourless (fig. CXII. 1); it assumes a reddish tinge only when aggregated in considerable masses.

1. A particle of the human blood as it appears when
transparent and floating; 2. the same dry, seen as opaque,
illuminated by a leiberkuhn; 3. the same as it appears
when half the leiberkuhn is darkened; 4. a particle of the
frog's blood floating; 5. the same seen on its edge. All
the above objects are magnified 500 diameters[5]