GENERAL CHARACTERISTICS OF THE BLOOD 163 



as nearly identical conditions as possible, because, inasmuch as the 

 internal friction of the blood varies with the size of the channel, its 

 content in solids must necessarily be subject to fluctuations. 



The specific gravity of the blood varies considerably. On the one 

 hand, it is found that the general body-fluid of the lower forms possesses 

 a density only slightly greater than that of water, and, on the other, 

 that the blood of the higher animals represents a complex fluid of 

 relatively high concentration. Obviously, therefore, the specific 

 gravity must increase steadily, the highest values being present in the 

 mammals. It is true, however, that the density of the blood of closely 

 related animals is subject to only slight variations, so that a rather 

 definite grouping of animals in accordance with this factor is made 

 possible. Thus we find that the specific gravity of human blood ex- 

 ceeds that of the blood of the dog or cat, and that these in turn surpass 

 that of the blood of the rabbit. The density of turtle's or frog's 

 blood is much less than that of the mammalian blood. It must be 

 emphasized, however, that individual variations do occur, as may be 

 gathered from the fact that the specific gravity of human blood varies 

 between 1.054 and 1.066. A fair average value is 1.060. In woman 

 variations between 1.054 and 1.061 have been noted. Blood serum 

 shows values ranging between 1.028 and 1.032. The red corpuscles 

 possess a much greater density (1.090), a fact which accounts for the 

 rather quick deposition of these bodies in blood to which an anticoagu- 

 lating agent has been added. 



It is also of interest to note that the specific gravity of the blood 

 of the fetus is usually higher than that of the blood of the mother. 

 Values, such as 1.066 for the former and 1.054 for the latter, are not 

 unusual. During the first few days after birth rapid fluctuations 

 between 1.060 and 1.080 are the rule, which, in all probability, are 

 caused by a more copious production of tissue-fluid. During the first 

 few months variations between 1.053 and 1.059 are encountered; a 

 fair average value at this time being 1.056. A slightly higher figure, 

 namely 1.058, is reached after the second and before the fourth year. 

 Subsequent to the sixth year the average value is 1.061. This is re- 

 tained throughout childhood. 



Similar fluctuations have been recorded in dogs, cats, rabbits, and 

 other animals. It may be stated, however, that the average value 

 for dog's blood is somewhat lower (1.055) than that of the blood of 

 man, but higher than that of the blood of the cat (1.050) or rabbit 

 (1.045). On the whole, therefore, it is true that the specific gravity in 

 a particular group of animals displays a certain constancy and that the 

 minor variations just alluded to are dependent in a large measure upon 

 such influences as age, sex, exercise, intake of solid food and water, as 

 well as loss of water by perspiration and otherwise. 1 



1 The reader is reminded of the fact that very profound changes in the specific 

 gravity are frequently encountered during pathological conditions. Very low 

 values are noted in anemias, and very high values in diseases which are character- 

 ized by an increase in the number of the red blood cells (polycythemia). 



