THE BLOOD. 



human blood; but in both the sloth and the elephant, they are larger. 

 If it were only required to decide whether a given specimen of fresh 

 blood belonged to man or to the musk deer, for example, or even to the 

 goat, no doubt the difference in size of the globules would be sufficient 

 to determine the question. 



But within nearer limits of resemblance it would be doubtful, because 

 the size of the red globules varies to some extent in each kind of blood ; 

 and in order to be certain that a particular specimen were human blood, 

 it would be necessary to show that the smallest of its globules were 

 larger than the largest of those belonging to the animal in question, or 

 vice versa. The limits of this variation have been tolerably well defined 

 for human blood, but not sufficiently so for many of the lower animals 

 to make an absolute distinction possible. 



In the examination of stains or blood spots, the difficulty is increased 

 by the fact that the drying and subsequent moistening of the globules 

 introduces another element of uncertainty as to their exact original size. 



Physiological Function of the Bed Globules. There is no doubt that 

 the red globules of blood serve mainly as the carriers of oxygen. The 

 extreme readiness with which they absorb this substance from the 

 atmosphere or from any other gaseous mixture containing it, their im- 

 mediate change of color depending upon the supply or withdrawal of 

 oxygen, corresponding with the change of color in the blood as it tra- 

 verses the lungs or the capillaries of the general circulation, all indicate 

 that they have a special relation to the introduction and distribution of 

 oxygen in the living body. As a general rule, in those animals where 

 the red globules are of large size and few in number, the activity of the 

 vital functions is below the average ; while in the species where they are 

 smaller and more numerous, the processes of respiration, circulation, 

 nutrition, and movement are increased in rapidity to a similar degree. 

 The strongly marked physical and chemical characters of the red glo- 

 bules correspond with their importance in the functions of vitality. 



White Globules of the Blood. 



Beside the red globules above described, the blood contains a certain 

 number of other cellular bodies, which differ from the former in several 

 important particulars. These are the white or colorless corpuscles. As 

 their name implies, they are destitute of red or other coloring matter, 

 but under many circumstances present under the microscope a glistening 

 appearance, and when collected in large quantity may give to the fluid 

 or clot which contains them a whitish hue. They are much less abun- 

 dant than the red globules, the average proportion in healthy human 

 blood being one white globule to 300 red. They are nearly spherical 

 in form, and measure, on the average, 11 mmm. in diameter. They are 

 accordingly, in human blood, distinctly larger than the red globules. 

 (Fig. 76, c.) As regards their structure, they consist of a soft, some- 

 what viscid, colorless, finely granular substance, containing in its 

 interior one, two, or three ovoid nuclei. They are less yielding and 



