CHAPTER IV 

 CIRCULATION, RESPIRATION AND EXCRETION 1 



i. CIRCULATION 



172. Distribution of nutrients. The digestive changes by 

 which the ingredients of the feed are prepared for the nutrition 

 of the organism take place outside the body proper (113). In 

 order that the products formed shall serve their purpose they 

 must not only be taken up into the body by the processes of 

 resorption described in the preceding chapter but they must 

 be distributed through it, so that each of its myriad cells may 

 receive the substances which it requires. The chief vehicle of 

 this distribution is the blood, into which the resorbed nutrients 

 are transferred, directly or indirectly, and the distribution is 

 accomplished by means of the circulation of the blood, dis- 

 covered by Harvey in 1621. 



173. The blood. This familiar but highly complex fluid 

 serves a variety of purposes, being not only the carrier of the 

 resorbed feed ingredients to the tissues and cells but transmitting 

 to them the equally necessary oxygen and carrying away the 

 products of their activity to be used in other parts of the body 

 or to be excreted. 



The blood of the higher animals is a thickish, somewhat viscid 

 fluid, having a faint but peculiar odor, a slightly salt taste and 

 a color varying from bright to dark red. It is somewhat heavier 

 than water (sp. gr. 1.045-1.075), and contains about 21 per 

 cent of total solids. Under the microscope it is seen to consist 

 of a clear fluid, the plasma, holding in suspension a vast number 

 of small, solid bodies, the corpuscles. The latter are of two 

 kinds, known as the red corpuscles, or ery throcytes, and the white 

 corpuscles, or leucocytes. 



1 Only such a very general consideration of the outlines of these functions as seems 

 essential for a proper comprehension of the phenomena of metabolism and of the 

 processes of nutrition is attempted here. For a more complete elementary discus- 

 sion, the reader is referred to Hough and Sedgwick's The Human Mechanism, 

 Chapters IX, X and XI, and for further details to the larger treatises on physiology. 



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