CHAPTER IV 

 THE BLOOD 



THE blood is the fluid medium of which all the tissues of the body are 

 nourished. By means of the blood materials absorbed from the alimentary 

 canal as well as oxygen taken from the air in the lungs are carried to the tissues, 

 while substances which result from the metabolism of the tissues are carried 

 to the excretory organs to be removed from the body. The blood also acts 

 as a medium of exchange for products of glandular activity between the various 

 tissues themselves, internal secretions, and it is a factor in the regulation 

 of body temperature. The blood is a somewhat viscid fluid, and in man 

 and in all other vertebrate animals, with the exception of two of the lowest, 

 is red in color. The exact color of the blood is variable; that taken from 

 the systemic arteries, from the left side of the heart and from the pulmonary 

 veins is of a bright scarlet hue; that obtained from the systemic veins, from 

 the right side of the heart, and from the pulmonary artery is of a much 

 darker color, which varies from bluish-red to reddish-black. At first 

 sight the red color appears to belong to the whole mass of blood, but on 

 further examination this is found not to be the case. In reality blood con- 

 sists of an almost colorless fluid, called plasma or liquor sanguinis, in which 

 are suspended numerous minute masses of protoplasm, called blood-corpus- 

 cles. The corpuscles are of the two varieties, the white ameboid corpuscles, 

 or leucocytes, and the red corpuscles, erythrocytes. The latter compose 

 by far the larger mass of blood-cells and contain the red pigment, hemoglobin, 

 to which the color of the blood is due. 



The plasma or fluid part of the blood is a remarkably complex chemical 

 mixture. It is kept in constant rapid circulation through the blood-vessels 

 of the body and is, therefore, thoroughly mixed and homogeneous in character. 



Quantity of the Blood. The quantity of blood in any animal 

 under normal conditions bears a fairly constant relation to the body weight. 

 The amount of blood in man averages -$ to YJ of the body weight. In 

 other mammals the proportion of blood is also fairly constant, varying from 

 ^ to -^g- of the body weight. In many of the lower vertebrates the relative 

 quantity of blood is very much less. 



An estimate of the quantity in man which corresponded very nearly 

 with this proportion has been more than once made by methods illustrated 

 by the following data: A criminal was weighed before and after decapita- 



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