3J6 CIRCULATION OF BLOOD. LECT. XVIl. 



blood. The quantity of it varies in different animals, and 

 there appears to exist a certain ratio between its weight 

 and that of the animal. Valentin has pointed out an inge- 

 nious method of determining its total quantity : it consists in 

 first ascertaining the percentage quantities of water and solid 

 matters contained in the blood drawn from an animal by 



Fig. 24. 



Red Corpuscles of Human Blood, represented, at a, as they are seen when rather 

 beyond the focus of the microscope ; and at b as they appear when within the focus. 

 Magnified 400 diameters. 



a small bleeding. A given weight of water is then to be 

 injected into one of these blood-vessels, and again we must 

 ascertain the percentage composition of the blood thus 

 diluted, that is, the proportion between the water and solid 

 constituents. It is easy to perceive how, with these data, 

 we may obtain the amount of the total quantity of the blood. 



It is assumed that man contains, on an average, from 

 12 to 15 kilogrammes [about 30J to 40 i troy pounds] of 

 blood. The ratio which exists between the weight of the 

 blood and that of the man will be about 1 to 5. 



Number of Pulsations. The heart of an adult man con- 

 tracts from seventy to seventy-five times in a minute ; but 

 the number of pulsations varies according to the age, sex, 

 temperament, and idiosyncrasies of individuals, the species 

 of animal, and the pressure of the atmosphere. Thus in 

 the new-born infant the number of pulsations is from a 

 hundred and thirty to a hundred and forty ; in fishes, from 



