CHAPTER IV. — DESTRUCTIVE METABOLISM 

 I. RESPIRATION 



Respiratory organs. — The word respiration, or its English equivalent, 

 breathing, suggests at once the currents of air into and out of the lungs, 

 and the bodily movements that cause them. The reason for this is that 

 so much attention has been given to these matters in human physiology 

 that the more important processes, which take place in the muscles and 

 live tissues generally, have been almost ignored. This is emphasized 

 by the fact that the phrase " respiratory organs " means the lungs and 

 the air passages thereto, while the blood, which is an equally important 

 adjunct to the aeration of the tissues, is not usually included. But air- 

 passages, lungs, chest wall, diaphragm, blood vessels, and blood, not to 

 mention others, are all necessary organs. The fundamental processes, 

 however, take place in the living cells; and they go on there, for a time 

 at least, whether or not, by accessory mechanical means ; the oxygen of 

 the air is supplied and the waste products removed. 



Since in plants the accessory organs are very simple indeed, their 

 structure and behavior needs little consideration, particularly as they are 

 at the same time, in green plants, related to transpiration and to photo- 

 synthesis (see aerating system, p. 318). So botanists have focused 

 attention upon the essential processes in respiration. This difference in 

 emphasis has tended to obscure the fundamental likeness of this function 

 in plants and animals.' 



Identical in plants and animeds. — Excluding the processes of aeration, 

 respiration in plants and animals is alike in all essentials. When the 

 likeness of the living matter in the two is considered — a likeness so 

 great that neither microscopic observation nor analysis can distinguish 

 them by structure, behavior, or composition — the fundamental identity 

 is not surprising. Yet popularly it is widely believed that the respira- 



' It has been proposed to retain the term respiration for the aerating processes, and to 

 use the term energesis for the chemical changes in the tissues, whose end seems to be the 

 setting free of energy. It remains to be seen whether or not this distinction is acceptable 

 or important. It may prove, indeed,.that the release of energy is quite incidental to other 

 more essential processes. 



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