APPENDIX — EXTERNAL RESPIRATION. 651 



the red colouring matter of the blood would be apt to stain 

 the litmus paper, and it would be almost impossible to say- 

 when the reddening was due to it, and when to free acid. In 

 order to prevent this, a quantity of chloride of sodium is 

 added to the acid. The salt prevents the corpuscles from 

 being dissolved, and the haemoglobin from disturbing the 

 reaction. The acid and salt are gradually added to the blood, 

 and the reaction tested from time to time by putting a drop 

 of the blood on a piece of fine satin paper coloured with litmus. 

 The paper should be first moistened with a tolerably strong 

 salt solution, the drop of blood allowed to remain on it for 

 a few seconds, and then wiped off with blotting paper. The 

 point of saturation is held by Schulte, who has also employed 

 this process, to be reached whenever the blue litmus paper 

 becomes distinctly reddened, even though the red colour 

 should disappear again immediately. This transient reddening 

 is due to carbonic acid ; and Schulte prefers it to the first 

 permanent reddening, because it can be more easily observed. 

 This does not give the absolute amount of alkalinity ; but all 

 that is wanted is the comparative alkalinity of the three 

 portions, and this is got accurately enough if they be all 

 treated in the same manner as regards temperature, shaking, 

 etc. In this way, Zuntz, Scharrenbreich, and Schulte, find 

 that quinine lessens oxidation in the blood, and Binz finds that 

 it does the same in a solution of haemoglobin. 



External respiration, or the interchange of gases between 

 the blood and the atmosphere, takes place whenever they come 

 into sufficiently close relation with one another, as they do in 

 the capillaries of the skin, intestinal canal, or lungs. In the 

 frog, respiration is carried on b}' the skin to such an extent, 

 that it can live for a considerable time after the lungs have 

 been excised ; but in mammalia respiration is carried on 

 almost entirely by these organs, and any interference with 

 their function quickly puts an end to the life of the animal. 

 In order that the blood which circulates through the body 

 may get rid of its carbonic acid and take up oxygen sufficient 

 for the wants of the tissues, fresh portions of it must con- 

 stantly be brought into contact with the air, and therefore 



