THE KIDNEYS 175 



n. Loss of Heat. • 



1. The skin. 



2. The lungs. 



3. The excretions by the kidney and the alimen- 



tary canal. 



B. SoTJRCES OF Gain :— 



I. Gain of Matter. 



1. The lungs : oxygen (fairly constant). 



2. The alimentary canal : food (variable). 



3. The tissues : products of their activity, waste 



matters (always going on but varying 

 according to the activity of the several 

 tissues). 



4. The lymphatics : lymph (always going on but 



varying according to the activity of the 

 several tissues).^ 



II. Gain of Heat. 



1. The tissues generally, especially the more 



active ones, such as the muscles. 



2. The blood itself, probably to a very small 



extent. 



2. The Kidneys.— In the preceding Lesson we have 

 described the operation by which the lungs withdraw from 

 the blood much carbonic acid and water, and supply 

 oxygen to the blood : we now proceed to the second source 

 of continual loss, the Kidneys. 



Of these organs, there are two, placed at the back of the 

 abdominal cavity, one on each side of the lumbar region 

 of the spine. Each, though somewhat larger than the 

 kidney of a sheep, has a similar shape. The depressed, or 

 concave, side of the kidney is turned inwards, or towards 

 the spine ; and its convex side is directed outwards (Fig. 

 50). From the niiddle of the concave side (called the 



1 The gain from those lymphatics which are caUed lacteals, since it 

 comes from the aUmentary canal, vanes much more. 



