The change in the constitution of the blood by which this separa tion into a solid and fluid portion is effected commences directly the blood leaves the blood-vessel. In about eight or nine minutes after blood is drawn from a living animal it begins to thicken, and in the course of a quarter of an hour the clot begins to form, and the serum exudes. This process arises from the fact that the fibrin is not dissolved, only suspended in the blood, and when allowed to stand it separates, sinking in the liquid blood, and carrying with it the blood-corpuscles. When the latter separate, from the fibrin, which they do under various circumstances, forming a layer at the lower part of the clot, the upper part of the clot, which is of a yellow or buff colour, is called the bully coat.
The Coagulation of the Blood is not simply a separation of the fibrin from the serum of the blood, dependent on physical causes, as is evident from the manner in which it is hastened or delayed by external causes. This may be stated without the necessity of making any inferences from the phenomena presented. Temperature exerts an influence, as cold delays coagulation, whilst moderate heat hastens its occurrence. Exposure to the atmosphere facilitates this process, as also contact with foreign bodies ; but the exclusion of air delays it. The cessation of active motion whilst the blood is in the body hastens coagulation, but movement also give. a tendency to it out of the body. A mixture of half the bey. of the blood with water increases the coagulative tendency, bus increased dilution diminishes it. States of the system affect it. Faintness is favourable to coagulation, but excitement and suffocation retard it. Coagulation is quicker in arterial than in venous blood. Foreign substances generally hasten it, but alkalies delay it.
The Clot or Crassamentum separates into portions—a substance of a yellowish-white colour forming the top of the clot, and a red mass always found at the bottom of the clot. When the yellowish substance forming the top of the clot is completely separated from the red mass it is found to be a solid of considerable consistence, soft, firm, elastic, and tenacious, or gluey. Its distinctive character is derived from the disposition manifested by its component particles to arrange themselves into minute threads or fibres ; these threads or fibres are often so disposed as to form a complete net-work. In its general aspect as well as in its chemical relations this substance bears a striking resemblance to pure muscular fibre ; that is, to muscular fibre deprived of its enveloping membrane and of its colouring matter.
Several names have been given to this substanee—gluten, coagulable lymph, fibre of the blood, and fibrin ; the latter is the name commonly appropriated to it. Of all the constituents of the blood Fibrin is by far the most important. Whatever other constituent may be absent, this in all animals which possess blood is invariably present The main part of all the solid structures of the body is composed of it ; it forms the bogie of muscle, and in the lower animals, in which distinct muscular fibres cannot be traced, it probably performs the function of muecle. This substance or some modification of it is also found in plants, and seems to bo the chemical compound with which the active functions of life are connected.
The second constituent of the Clot, the red matter, being heavier than the fibrin, gradually subsides to the lower surface, where it is always found forming the bottom of the clot. The proportion of this red matter to the fibrin differs in different classes of animals, and even in the same animal at different times. The greater the energy and activity of the animal the larger in the proportion of the red matter, and it is also generally largo in proportion to the elevation of the animal temperature.
When a drop of blood is placed under the microscope it in found to consist of the liquor sangumis and a number of globules or cella. It is these latter which constitute the red matter of the blood. When carefully examined those cells are found to be of two kinds—the one white or colourless, the other red. The former except in states of disease are far less in number than the latter, and are found to be identical with cells which are found in the lymph and chyle. Hence they are sometimes called lymph- or chyle-corpuscles Them white corpuscles have only of late years attracted much attention, though they had been described as far back as the time of Howson. In man and the mammalia they are often larger than the red corpuscles; they may be recognised by their granular appearance, their peculiar contour, and the irregular shading of their figure. (Figs. 4 and 5.) nucleus like those of birds, and they are not much more than half the size of even the smallest that have been observed in birds or reptiles.