bright scarlet colour in the arteries, but in the veins it is of a deep purple hue. The fundamental difference between venous and arterial blood consists in the relative proportion of the oxygen and carbonic acid gases contained in each. The proportion of oxygen being greater in arterial than in venous blood. The gases of the blood are oxygen, carbonic acid, and nitrogen. About 50 or 60 volumes of the gases collectively can be obtained from 100 volumes of blood. The average composition of the gas obtained from 100 volumes of blood may be set down as follows: The temperature of the blood is generally about 1000 F., but it varies considerably in different parts of the body, becoming warmer as it traverses the internal organs, especially the liver, muscles, and the nerve centres, but being cooled somewhat as it passes through the capillaries of the skin. The quantity of water present in the blood also varies greatly. It is increased when a quantity of fluid is taken, and when the organism is deprived of solid food. It is diminished by exercise and excessive action of the skin and kidneys. The fœtal blood is said to contain less water than the maternal. Blood has a peculiar odour, and it is said that it is possible to tell the animal from which a particular specimen of recently drawn blood is derived by the distinctive odour it possesses: the well-known odour of the pig and the cat, and the milky odour of the cow, being readily detected in the blood of these animals. When blood is withdrawn from the body it becomes sticky, and exhibits a marked tendency to cling to the vessel in which it is placed. This stage, which is known as the viscid stage, lasts from one to three minutes, it then passes into the jelly stage; and during this stage, if the vessel in which it is contained is inverted, and the clot shaken out, it will be seen to have taken a complete mould of the vessel it occupied. This stage lasts from three to eight minutes. Little beads of moisture now gradually make their appearance on the surface of the clot, and these gradually coalescing the surface of the clot is covered by a thin layer of strawcoloured fluid. The fluid now gradually appears between the sides of the clot and the wall of the vessel, and ultimately between the under surface of the clot and the bottom of the vessel, so that the clot floats in the straw-coloured fluid or serum which has been expressed from it. This last stage, the contraction of the clot and the separation of the serum, may go on for from twenty-four to forty-eight hours after coagulation has first set in. From this brief description it will be seen that, during the process of coagulation, the fluid part of the blood or liquor sanguinis separates into two parts: namely, into a firm solid part the fibrin, and a pale yellowish fluid the serum. When a considerable quantity of fibrin has become developed it undergoes contraction, and, involving the corpuscles in its meshes, it forms the firm part of the clot from which the serum is gradually expressed, as described above. The relation between the constituents of the blood in the liquid and It is very difficult to obtain any exact knowledge of the composition of the liquor sanguinis, as it is constantly undergoing changes from the time when it is first withdrawn from the body. The composition of the blood, in 1000 parts, is well given by Ralfe in his Physiological and Pathological Chemistry. |