THE BLOOD 231 



and known as the blood-platelet or plaque. 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 be roughly separated by appropriate means when the blood is withdrawn 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 cor- 

 puscles 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 hematocrit. 



PHYSICAL PROPERTIES OF BLOOD 



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

 the arterial, the color of which is a bright scarlet-red, 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 red corpuscles of 

 coloring-matter, hemoglobin, in 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 

 venous blood passes into and through the pulmonic capillaries the hemo- 

 globin absorbs a certain volume of oxygen after which it changes in color 

 and on emerging from the lungs imparts to the blood its characteristic 

 scarlet-red color. By reason of the union of the hemoglobin with the 

 oxygen it is generally termed while in the arteries, oxyhemoglobin. As 

 the arterial blood passes into and through the systemic capillaries, the oxy- 

 hemoglobin yields up a portion of its oxygen to the tissues after which it 

 again changes in color and on emerging from the tissues imparts to the 

 blood its characteristic bluish-red color. By reason of the loss of a portion 

 of its oxygen, the hemoglobin is generally termed while in the veins, deoxy- 

 or reduced hemoglobin. 



2. Opacity. Owing to the fact that the corpuscles have a refracting 

 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-matter is 

 evident 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, at once becomes transparent. 



3. Odor. When freshly drawn the blood possesses a peculiar charac- 

 teristic odor which has been attributed to the presence of a volatile fat acid 

 in combination with an alkaline base. The intensity of the odor may be 

 increased by the addition of concentrated sulphuric acid, by means of which 

 the volatile acid is set free. 



4. Specific Gravity. The specific gravity of blood lies within the limits 

 of 1.051 and 1.059, averaging in man 1.056 and in woman 1.053. Normally, 

 variations from these values are only temporary and are connected with 



