34 



SPECIFICITY OF THE BLOOD OF VERTEBRATES 



per cent, and in the latter 6 per cent, so that in the human being the pro- 

 portion is but little more than in an animal comparatively low in the scale 

 of life and in which the metabolic processes in comparison with those of 

 man are at a comparatively low level. A partial explanation of this incon- 

 sistency becomes at once obvious in the differences in the content of the 

 bloods as regards the constituents just referred to, the blood of the former 

 containing about 12.5 to 13.5 per cent of hemoglobin, and about 7.6 per 

 cent of proteins in solution in the plasma, while in the latter there are 

 about 2.5 to 3 per cent of hemoglobin and 2.54 per cent of plasma-proteins. 

 Likewise, mammals generally have a lower proportion of blood than birds 

 generally, but the percentages of hemoglobin and plasma-proteins are 

 notably higher in the former. The blood of all animals having nucleated 

 corpuscles, if not actually poorer in erythrocytes than the blood of mam- 

 mals, is usually or invariably poorer in hemoglobin and proteins. The 

 meanings of the differences in the proportions of blood in different genera, 

 etc., are as yet undetermined; but it seems that the proportion of blood in 

 relation to body-weight, the proportions of vital constituents of the blood 

 in relation to body-weight, and the rapidity with which the total volume 

 of blood is driven through the vessels, should collectively show a definite 

 and close relationship to the individual's position in the scale of life and 

 to the intensity of its metabolic processes. 



TABLE 10. Mean specific gravities deduced from the records of table 9. 



THE SPECIFIC GRAVITY OF THE BLOOD IN RELATION TO GENERA. 



So many conditions, especially age, diet, general nutritive state, 

 parturition, etc., may affect to even a marked extent the specific gravity 

 of the blood, that decided variations must be expected not only among 

 individuals of the same species, but also in any given individual from day 

 to day and hour to hour. Notwithstanding the difficulties of obtaining 

 accurate data under such conditions, the results of the investigations of 

 Lloyd Jones and of Sherrington and Copeman and others (Davy, Researches 

 Anatomical and Physiological, London; Lloyd Jones, Journal of Physi- 

 ology, 1887, vm, 874; 1891, xn, 299; Pfliiger, Archiv f. ges. Physiologic, 



