If the tubes are charged in the first place with the blood of the horse, ox, or other animal, the corresponding blood is coagulated, while that of any other animal dissolves. In this way it is possible to apply the physiological test without the necessity of preparing a special serum by inoculation. * * * * * From time to time in criminal trials, the latest instance being in the Crippen case, the question occurs whether a given specimen of hair is of human origin or has been derived from an animal. Thanks to the pronounced difference in appearance shown by hairs of different origin when viewed under the microscope there is no difficulty in giving a positive answer to this question. Human hair is characterised by being fairly uniform in diameter throughout most of its length and then tapering gradually to a fine point. The hair of an infant has very few scales upon its surface, and these stand out prominently, but in the case of an adult the scales are very numerous and appear closely pressed against the axis of the fibre. Another peculiar point of difference between the hair of a young child and that of a full-grown person is that in the case of the former there are some particulars in which the hair resembles that of certain animals. Thus it has a jointed appearance recalling to some extent the structure of the fibres of merino wool. In the hair of many animals the medulla, or central canal, is plainly visible under the microscope, but such medullated fibres are apparently not formed in the case of human hair. As the hair of many domestic animals might on superficial examination be mistaken for human hair, it is essential to take note of the characteristic differences, some of which are shown in the accompanying figures. Three types of hair are found upon the cow, viz.: thick beard hairs, showing a medulla, soft woolly hairs, and fine beard hairs, both of which are without a medulla. In those fibres in which it is present the medulla is very pronounced and tapers towards the apex. The hair of the calf has the same structure as that of the cow. Horse-hair is characterised by its lustrous cylindrical appearance. The commercial fibre is mainly derived from the mane and tail, and is much thicker and stiffer than the hairs from the body, which are those most likely to be met with in criminal investigations. As a rule, the latter are less than an inch in length, and the medullary canal is well marked. In rabbit's hair the medulla is also very pronounced and is characterised by its structure of curious quadrilateral cells, which may either form a single row or increase to four or eight rows as the hair becomes wider. On the surface of the hair are numerous scales which fit into one another after the manner of the joints in a bamboo cane. The chief commercial use of the rabbit's hair, which is usually about half an inch in length, is the manufacture of linings for hats.