Bespiration and Cell Energy. 
417 
alveoli that the interchange of gases takes place, the walls being closely 
reticulated with fine blood capillaries. The air tubes are all connected 
with a main tube or bronchus which is connected with the mouth. In 
addition the main bronchus also gives off branches which end in a series 
of thin-walled air sacs which lie in the body cavity and are in communica- 
tion with the pneumatic cavities of the bones. 
In mammals the structure of the lungs is somewhat similar to that of 
birds, but the lungs are more distensible and are separated from the 
viscera of the body by a transverse muscular partition called the 
diaphragm, which helps in the mechanical part of breathing. In some 
mammals in addition to the excretion by the kidneys some of the de- 
composition products of metabolism are removed by the agency of the 
skin through the process of perspiration. 
In conclusion, it should be noted that respiration is not a function of 
any particular organ of a body but is a process going on in each individual 
cell. Thus the lungs should not be called the organs of respiration — they 
are only accessory organs. The kidneys might just as well be considered 
as further organs of respiration in so far as they remove the decomposition 
products indirectly from the respiring cells, and the heart in so far as it 
pumps the respiring medium through the body. 
Summary. 
1. The energy required in the metabolism of colourless cells is not 
obtained from the sun either directly or indirectly. 
2. The protoplasm in colourless cells only uses energy set free by some 
chemical union taking place either in the cell or by the introduction of 
external elements into the cell. In no case is energy obtained by the 
decomposition of substances in the cell, as indeed energy is required to 
bring about such decomposition. 
3. Energy is not required for synthesis of compounds for which 
chemical affinity is responsible, although undoubtedly chemical affinity 
is inextricably connected with energy. 
4. No plant substance contains a store of directly available energy. 
5. Eespiration is not a process apart from nutrition. The term 
respiration should be used exclusively to mean the interchange of gases 
taking place in each individual cell. 
6. That water is the respiratory medium for a large number of the 
living cells in trees. 
Statements in Pfeffer's "Physiology of Plants," vol. i., which, 
according to this paper, cannot be considered correct : — 
Page 295. " Carbon compounds provide both energy and material for 
growth, so that, &c," 
