STAINS OF BLOOD. Blood Is the vital fluid of the body, consisting of certain cellu lar elements suspended In serum or plasma. It carries nutriment to, and waste products from, the tissues, is freshened or aerated by Its passage through the lungs, and possesses the peculiar quality of spontaneous clotting.
Blood stains. are the red-brown color left by the contact of blood with any absorbent material, or the evaporated residue of blood left on non-absorbent surfaces and consisting of the cellular elements of. the blood and the dried albuminous matters.
The blood cells can at times be recovered more or less intact, if the stained material is soaked or washed in a solution of ordinary salt in the strength of .85 grammes to 100 cubic centimetres of water (normal salt solu tion, the normal salt concentration of the hu man blood, in which blood cells retain their color and form indefinitely). Blood cells re moved in this manner from a blood stain can be recognized under the microscope, and cer tain animal bloods can be -distinguished by their form and measurements. The principle of hremolysis can be employed to determine whether the blood cells are those of a human being or of certain animals. This is a re markably specific test, depending upon the fact that the blood of one species of animal, introduced into the tissues or circulation of an animal of another species, will cause the blood cells of the latter animal to disinte grate or hremoIyze. In certain cases the blood cells of one species (say A) are not af fected by the blood serum of another (say B); following the principles of immunity, as seen in a vaccination, it is4ound that if the blood cells of A are repeateldly injected into the body of B, there develops in the B ani mal's blood a body (the immune body) which will now quickly disintegrate the blood cells of the animal A. The specific reaction has
been developed. For the identification of hu man blood, rabbits are prepared by repeated ly injecting them with small amounts of hu man blood cells. In the rabbit blood-serum, there develops the specific immune body which, when brought in contact with any human blood cells, will dissolve or hnmolyze them. The blood cells of these animals re main undissolved.
If the blood stains will not yield formed: blood elements, they are put into solution, water or in the above described salt solution, and several tests can be made: The spectro scopic; the test for crystals ; the test for the presence of haemoglobin by certain oxyl dizing chemicals. The first two, when post tive, can be relied upon as showing that the dissolved stains contain blood coloring mat ter ; but they will not distinguish human from animal blood.
The third test is unreliable, and though used in many laboratories, is found not in frequently to show the reactions with many other quite foreign substances. See McWeen ey (Proc. Royal Acad. Med., Ireland, 1910); Muir's Studies on Immunity (Oxford, 1909).