FORENSIC MICROSCOPY. 129 



their recognition is not easy. In water, for example, 

 the corpuscles swell up to a spherical form. The com- 

 monest objects which may be mistaken for blood-cells 

 are the spores of certain plants, and medico-legal litera- 

 ture contains ludicrous instances of erroneous expert 

 opinions based on such findings. With a good prepara- 

 tion carefully compared with specimens of fresh blood, 

 such errors can easily be avoided; and, since the cor- 

 puscles of birds are of oval shape and show a distinct 

 nucleus, blood-stains of avian origin may be excluded. 

 The blood of other mammals can be differentiated from 

 that of man only by the size of the cells, and the differ- 

 ences are so slight that a certain result can seldom be 

 reached. The blood of the goat has cells less than 4.5 // 

 in diameter, while the corpuscles of the horse and cow 

 and pig are under 6 fi; but the corpuscles of the dog, 

 rabbit, mouse, cat, and other domestic animals very 

 closely resemble those of human blood. Such differ- 

 ences of one or two micromillimeters cannot be relied 

 upon, since even in the fresh blood of a living animal 

 some variation occurs and distorted dried specimens 

 are much more dubious. Dr. E. L. Walker has pointed 

 out that the white blood-cells of different mammals differ 

 much more widely than do the red corpuscles; and the 

 study of these bodies may furnish valuable aid to the 

 microscopist. 



The crucial test for blood-stains, and the only method 

 by which the blood of man may be definitely distinguished 

 from that of other mammals, depends upon a specific 

 biologic reaction similar to that which is used in the 



