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. 1 
Identical in plants and animals. 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- 
1 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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