230 TEXT-BOOK OF PHYSIOLOGY. 



tides termed corpuscles floating in it, of which there are two varieties, 

 the red and white, including among the latter two smaller bodies, the 

 blood-plates and elementary granules. The different constituents 

 can be roughly separated by appropriate means when the blood is 

 withdrawn from the body. If the blood of the horse is allowed to 

 flow directly into a tall cylindric glass vessel, surrounded by ice, it 

 separates in the course of a few hours into three distinct layers in 

 accordance with their specific gravities. The lower layer is dark red ^ 

 and consists of the red corpuscles; the middle layer is grayish in color \ ^> 

 and consists of the white corpuscles; the upper layer is clear and-^ 

 transparent and consists of the plasma. The red corpuscles occupy \ 

 almost one-half, the white one-fortieth, the plasma a trifle more than 

 one-half of the height of the entire blood-column, which indicates 

 approximately the different volumes of each. The same result can 

 be obtained with human blood by the use of the centrifuge or hema- 

 tocrit. 



PHYSICAL PROPERTIES OF BLOOD. 



1. Color. Within the blood-vessels two kinds of blood are dis- 

 tinguished the arterial, the color of which is a bright scarlet, and 

 the venous, the color of which is a dark bluish-red or purple. The 

 cause of the color as well as the difference in color is. the presence in the 

 recTcorpuscles of a coloring-mater, hernqglobin, m different degrees of 

 combination with oxygen. The intensity of the color in either kind 

 of blood is dependent on the thickness of the blood-stream, for in the 

 finest capillaries, as seen under the microscope, there is an almost 

 total absence of color. As the arterial blood passes into and through 

 the systemic capillaries, the hemoglobin yields up a portion of its 

 oxygen to the tissues and changes in color, though the change is 

 not appreciable by the eye. On passing into the veins, however, 

 the blood-stream soon presents its characteristic dark bluish color, 

 which deepens as it approaches the lungs. On passing into and 

 through the capillary vessels of the lungs the hemoglobin absorbs a 

 new volume of oxygen, changes in color, and on emerging from the 

 lungs the blood presents its characteristic scarlet color. 



2. Opacity. Owing to the fact that the corpuscles have a re- 

 fracting power different from the plasma, the blood, even in thin 

 layers, is opaque. The repeated refractions and reflections which 

 light undergoes in passing through plasma and corpuscles is attended 

 by such a dissipation that it is impossible to see printed matter through 

 it. ^That the opacity is due to the shape of the corpuscles rather than 

 to their contained coloring-mattej^sevident from the fact that when 

 the hemoglobin is caused to separate from the corpuscles by the 

 addition of chemic reagents, the blood, though it deepens in color, 

 becomes at once transparent^ 



