CHAPTER XII. 



THE DIGESTIVE ORGANS AND THEIR 

 ANATOMY. 



In the discussions in the preceding chapters the blood 

 was made the source of all of the elements needed for the 

 tissues or secreted by the glands. In the chapter on respi- 

 ration alone was it pointed out that in turn the blood derived 

 its source of oxygen from the air, and that it derived its 

 supply of carbon dioxide from the tissues. In several chap- 

 ters there is now to be pointed out in what manner the 

 blood, which is the medium between the external world and 

 the tissues of the body, derives its supply to enable it in 

 turn to supply the tissues. 



The necessity for such a food supply is entirely too evi- 

 dent to need further comment. An organ stops work soon 

 after its supply of blood is cut off. But the blood is in no 

 sense a living tissue; has no vitality of its own; is, as far 

 as the tissues are concerned, a foreign body; is nothing 

 more, in short, than the circulating store-house out of 

 which, as it passes along, the hungry tissues may pick up 

 what they need for their own life. To cut the nutritive 

 value.of the blood down below a certain average composition, 

 or to put into the blood injurious substances will at once re- 

 act upon the tissues which derive their supplies from it. 

 But there are very few bodies which can at once be carried 

 by the blood and serve as nourishment for the live tissues. 

 Nearly all foods are in a solid state, and in this condition 

 they are unable to pass into the circulation. Even many 

 foods in a liquid condition to begin with, are nevertheless 

 not available as foods when directly placed in the blood. 

 Thus, the albumen of an egg, certainly one of the most 

 nutritious of foods, is to some extent poisonous when in- 

 (264) 



