CHAPTEE XII. 



THE BLOOD. 



THE blood, in its natural condition, while circulating in the vessels, 

 is a thick opaque fluid, varying in different parts of the body from a 

 brilliant scarlet to a dark purple or nearly black color. It has a slightly 

 alkaline reaction, and a specific gravity of 1055. It consists, first, of a 

 nearly colorless, transparent, alkaline fluid, termd ihQ plasma, containing 

 water, fibrine, albumen, and salts, in a fluid condition; and, secondly, of 

 a large number of distinct cells, or corpuscles, the blood-globules, swim- 

 ming freely in the liquid plasma. The globules form about 40 per cent., 

 and the plasma about 60 per cent., by volume, of the entire mass. The 

 specific gravity of the two ingredients is somewhat different. That of 

 the plasma is about 1030 ; that of the globules, 1088. Their relative 

 quantities by weight are therefore more nearly equal to each other than 

 when estimated by volume; the exact proportions, according to Robin, 

 being nearly 45 per cent, of globules and 55 per cent, of plasma. 



Notwithstanding the difference in specific gravity between the blood- 

 globules and the plasma, the natural movement of the blood in the 

 vessels keeps them thoroughly mingled ; and even when the blood is 

 allowed to remain at rest in a glass jar, the globules subside only very 

 slowly and imperfectly. Thus the globules, disseminated uniformly 

 throughout the plasma, give to the entire mass of the blood an opaque 

 aspect and a deep red color. 



The globules of the blood are of two kinds, namely, red and white; 

 of these the red are by far the most numerous. 



Red Globules of the Blood. 



The red globules of human blood are so abundant that, in the thinnest 

 layer under the microscope, they appear crowded together in such profu- 

 sion as to cover or touch each other in every direction. According to 

 the estimates of Welcker and Vierordt about 5 millions of them are con- 

 tained in each cubic millimetre of blood. On account of their quantity 

 therefore, as well as their peculiar properties, it is evident that they 

 form a most important constituent of the circulating fluid. 



Physical Properties of the Red Globules. The red globules of hu- 

 man blood present, under the microscope, a perfectly circular outline 

 and a smooth exterior. According to the most recent and careful mea- 

 surements of various observers, they have, on the average, a transverse 

 diameter of from 7.50 to 7.75 mmm. Their size varies more or less, 

 but this variation is not very marked for the greater number of the 



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