KATABOLISM. .j- 



but are manufactured out of blood and used in the 

 economy of the animal mainly in the preparation of 

 food. In the latter the products pre-exist in the blood, 

 are poisonous to the blood, and must be removed. In 

 the former there is first manufacture and then elimina- 

 tion of a useful product. In the latter there is simple 

 elimination of a hurtful product. Salivary glands, pep- 

 tic glands, pancreas, and mammary glands are examples 

 of secretory organs. The lungs and the kidneys are 

 the best examples of excretory organs. The liver is pe- 

 culiar and of a mixed character. The pure secretory 

 organs connected with the process of food preparation 

 we have already discussed. They are concerned with 

 anabolic processes. We are now concerned with the 

 purely eliminative or excretory organs, for these belong 

 to katabolism. The liver, being mixed in its functions, 

 will be taken up later. By far the most important of all 

 the katabolic processes is that of respiration. 



SECTION II. 

 Function of Respiration. 



We have already given the morphology of the respira- 

 tory organs, because this could not be separated from an 

 account of the circulation. But the physiology of respi- 

 ration — i e., its function in animal economy, its relation 

 to katabolism — is the same in all animals and must be 

 taken now. 



We have already seen (page 358) that the general 

 purpose of. respiration is the aeration of the blood, or, 

 more specifically, the exchange of C0 3 of the blood for 

 oxygen of the air. Thus much it was necessary to assume 

 in order to understand the course of the circulation and 

 the changes in the blood in that course. We must now 

 explain the essential nature of the function and its neces- 



