As previously stated, the examination of alleged blood-stains from a medico-legal standpoint is pursued essentially for the purpose of testifying as to whether they have been produced by blood or not. Where the examination yields a negative result, further procedure is necessary with a view of identifying the true nature of the stain. Should, however, the result be positive, the question arises as to the possibility of distinguishing between human blood and the blood of other animals, and determining the exact animal from which the blood has been derived. Such an examination should be pursued in full recognition of its importance as a factor towards the establishment of truth essential to the administration of justice. To fulfil this obligation the methods employed should be so selected as to produce results bearing testimony free from any possibility of doubt. Certain differences exist, and may be detected by microscopical examination, between the red corpuscles of human blood and those of some other animals sufficiently well marked to render differentiation possible. The differences are those of form and structure. (1) In _man_ the red corpuscles appear as _circular_ biconcave discs, averaging ¹/₃₅₀₀ of an inch in diameter, and are non-nucleated. The red corpuscles of mammals present the same features, with the exception of the (2) _Camel_ tribe, in which the corpuscles are _oval_ in form, but non-nucleated. (3) In _birds_, _fishes_, _reptiles_, and _amphibians_ the red corpuscles are oval in shape, and possess a nucleus. Guided by the above facts, one is able to testify whether or not the corpuscles exhibit the characters of mammalian blood. Many attempts have been made with a view to establishing a reliable means of differentiation between the red blood corpuscles of man and other mammals (the camel excepted), and with a certain degree of success, such as might be expected, under select conditions favourable to histological research, but which do not obtain in medico-legal practice. Differences in size of the red corpuscles, as revealed by micrometric measurement, have been suggested as a possible means of distinguishing between the blood of different mammals. Of the common animals, the red blood corpuscles of the sheep present the most marked difference in size compared with those of man. The following table of the dimensions of red blood corpuscles is derived from measurements made by Treadwell, and quoted by White (_The Medico-Legal Journal_, New York, 1895): μ. Human 7.940 Dog 6.918 Rabbit 6.365 Ass 6.293 Pig 6.101 Horse 5.503 Cat 5.463 Ox 5.436 Sheep 4.745