LESSON IV 

 RESPIEATION 



1. The Gases of Arterial and Venous Blood. — The 



blood, the general nature and properties of which have 

 been described in the preceding Lesson, is the highly 

 complex product, not of any one organ or constituent 

 of the body, but of all. Many of its features are doubt- 

 less given to it by its intrinsic and proper structural 

 elements, the corpuscles ; but the general character of 

 the blood is also profoundly affected by the circumstance 

 that every other part of the body takes something from 

 the blood and pours something into it. The blood may 

 be compared to a river, the nature of the contents of 

 which is largely determined by that of the head waters, ' 

 and by that of the animals which swim in it ; but which 

 is also very much aflected by the soil over which it 

 flows, by the water-weeds which cover its banks, and 

 by affluents from distant regions ; by irrigation works 

 whicli are supplied from it, and by drain-pipes which flow 

 into it. 



One of the most remarkable and important of the 



