218 FUNCTIONS OF NUTRITION. 



Diagnosis of Blood, and the distinction between Human Blood and 

 that of Animals. It is often of consequence to recognize blood in 

 various animal fluids in physiological experiments, and it sometimes 

 becomes important in medico-legal investigations. For this purpose, in 

 the fresh fluids, nothing can be more satisfactory than spectroscopic ex- 

 amination ; a very small quantity of hemoglobine being sufficient to 

 yield a spectrum with the characteristic absorption bands. This method 

 has the further advantage that it enables us to detect the presence of 

 blood where its globules have been dissolved or their coloring matter 

 reduced to a fluid condition. The washings of a blood stain may show 

 the spectrum of hemoglobine, although they may not contain any red 

 globules perceptible by the microscope. This, however, only shows the 

 presence of the coloring matter of blood, and allows us to distinguish it 

 from other colored fluids ; it does not distinguish between the blood of 

 man and that of animals, since the hemoglobine is the same in all. 



But by microscopic examination of the red globules, either when 

 fresh or after having been dried and again moistened, we can often dis- 

 tinguish the blood of an inferior animal from that of man. According 

 to Richardson,* a fragment of dried blood, weighing less than y^ of 

 a milligramme, which has been kept for five years, if decolorized with a 

 weak watery solution (0.75 per cent.) of sodium chloride, and afterward 

 tinted with aniline, may exhibit the blood-globules in such a condition 

 that their size can be accurately measured. 



If a blood stain, accordingly, which in watery solution gives the 

 spectrum of hemoglobine, be found to contain oval nucleated globules, 

 it must be the blood of a bird, reptile, or fish ; and the oval form alone 

 would show that it is not human blood. The question whether a speci- 

 men be composed of human blood may consequently be decided in the 

 negative by microscopic examination. But if the specimen contain cir- 

 cular globules, without nuclei, it will be impossible to say whether they 

 belong to human blood, or to that of some animal, such as the ape or 

 the dog, whose globules nearly approach the human in size. In most 

 domesticated quadrupeds, the globules are smaller than in human blood ; 

 while in both the sloth and the elephant, they are larger. If it were 

 only required to decide whether a specimen of blood belonged to man, 

 or to the elephant or the musk deer, for example, or even to the goat, 

 no doubt the difference in size of the globules would be sufficient to 

 determine the question. 



I But within nearer limits of resemblance it would be doubtful, because 

 i the size of the globules varies to some extent in each kind of blood ; 

 ' and in order to be certain that a particular specimen were human blood, 

 it would be necessary to show that the smallest of its globules were 

 larger than the largest of those belonging to the animal in question, or 

 vice versa. The limits of this variation have been tolerably well de- 

 fined for human blood, but not sufficiently so for many of the lower 

 animals to make an absolute distinction possible. 



* Monthly Microscopical Journal. London, September 1, 1874, p. 140. 



