_Menstrual blood_ contains no fibrin, has an acid reaction due to the vaginal mucus which keeps it fluid, and contains squamous epithelial cells. None of these characters can be differentiated on fabrics, especially when contaminated with urinary stains in addition. Hence, in cases of alleged rape, no distinction can be drawn between blood-stains on the underclothing of the female, which may have arisen from hæmorrhage the result of violence to the sexual parts, and those which might have arisen from the ordinary menstrual flow or metrorrhagia. The detection of spermatozoa, however, would add considerable value to the observation. [Illustration: Fig. 8.--Measurement of Blood Corpuscles. Photo-micrograph of human red blood corpuscles, × 800. Each corpuscle in diameter covers two divisions of the scale. Compare with sheep‘s blood, Fig. 9. (R. J. M. Buchanan.)] [Illustration: Fig. 9.--Measurement of Blood Corpuscles. Photo-micrograph of red blood corpuscles from the sheep, × 800. The diameter of the corpuscle covers one division of the scale. Compare with human blood, Fig. 8. (R. J. M. Buchanan.)] =Blood Crystals.=--Professor Preyer of Jena pointed out many years ago that the hæmoglobin crystals from the blood of some animals differed in shape from those of man, and this fact has given rise to many attempts to trace the identity of the blood to the animal from which it has been derived. The results have not been of sufficient value to establish it as trustworthy for medico-legal purposes. Dr. Monckton Copeman (_B. M. J._, vol. ii. p. 190, 1889) has carefully investigated the subject, and his researches, partly confirmed by Professor Glaister of Glasgow, show that from the guinea-pig, rat, and squirrel, crystals of hæmoglobin may be easily obtained, but the solubility of human hæmoglobin renders it much more difficult to crystallise. Crystals may, however, be obtained in the following ways: (_a_) By feeding leeches on human blood, crystals may be found, after some weeks, in the gastric dilatation of the alimentary canal. (_b_) By diluting human blood with the fluid from hydrocele, ascites, or pleurisy when they have undergone decomposition. (_c_) By adding crystals of glycocholate or taurocholate of soda to human blood. (_d_) By adding a drop of cat‘s bile to human blood on a microscope slide, but the crystals are those of reduced hæmoglobin. Crystals of human hæmoglobin appear in the form of rectangular plates, with a greenish or pale claret colour. On spectroscopic examination they exhibit the characters of reduced hæmoglobin, in contradistinction to the crystals derived from the lower animals, which produce the spectrum of oxyhæmoglobin.