CHAPTER III. 

 THE BLOOD. 



THE blood 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, the plas- 

 ma, containing water, albuminous matters, and salts, in solution ; and, 

 secondly, of distinct corpuscles, or blood globules, swimming in the 

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

 about 60 per cent, by volume, of the entire mass. The specific gravity 

 of the two 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 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 plasma, the natural movement of the blood in the circu- 

 lation keeps them thoroughly mingled ; and even when it is allowed to 

 remain at rest, the globules subside very slowly and imperfectly. Thus 

 the globules, uniformly disseminated through the plasma, give to the 

 blood an opaque aspect and deep red color. 



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

 the red are 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 cover or touch each other in every 

 direction. According to the estimates of Welcker and Vierordt about 

 5 millions are contained in each cubic millimetre of blood. On account 

 of their quantity therefore, as well as their properties, 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 circular figure and a smooth 

 exterior. According to the most recent measurements, they have, on 

 the average, a transverse diameter of from Y.50 to 7.75 mmm. Their 

 size varies more or less, but this variation is not very marked for the 

 greater number, and, according to Schmidt, over 90 per cent, of those 

 contained in a single specimen have the same dimensions. The smallest 

 size observed is 4.50 mmm. (Harting), and the largest 9.3 mmm. ; while 



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