57 



XV 



220. Blood. The blood is a red, thick, opaque fluid. 

 The specific gravity varies from 1.04S to 1.075 with an aver- 

 age for adult human beings of about 1.055 ; it depends pri- 

 marily upon the amount of hemoglobin present. 



For examination, it is convenient to consider the blood as 

 composed of two parts : the corpuscles and the albuminous 

 liquid in which they are suspended — the plasma. The solid 

 blood corpuscles in man may constitute nearly one-half the 

 weight of the blood. In some animals as the ox, they make 

 up but one-third of the weight of the blood. 



The color of the blood is caused by the red corpuscles. 

 Even comparatively thin layers of the blood are opaque from 

 their presence. The coloring matter (hemoglobin) can be set 

 free from the corpuscles by water or by many chemical 

 reagents. The color then becomes much darker, since the 

 li^ht is no longer reflected from the surface of the corpuscles. 

 The addition of strong neutral salt solutions to blood turns 

 it bright red, because of the increased reflection of light from 

 the shriveled corpuscles. 



221. Fresh blood may be obtained and defibrinated at a 

 slaughter house, and a few drops of formalin added- to it will 

 prevent putrefaction for some time. It is better, however, 

 when possible, to obtain the blood by bleeding an animal. 

 After the dog, or any other animal of convenient size, has 

 been anesthetized, the carotid or femoral artery is exposed 

 and isolated from surrounding parts for an inch or two of its 

 length, a-nd a clamp or ligature applied to the proximal por- 

 tion of the artery, i. e., as far as possible toward the heart. 

 A little distal to the clamp make an incision in the artery and 

 insert a glass canula and tie it tightly in place. Remove the 

 clamp or ligature and the blood will pass through the canula, 

 and tke animal allowed to bleed to death. When the animal 

 is apparently dead, an interesting experiment may be per- 

 formed by injecting into the artery some normal salt solution 

 of the same temperature as the body and note the reviving 

 effect. 



