16 ORGANISM AND ENVIRONMENT 
The effects in man of undue deficiency or undue 
excess of CO, can easily be observed. . By forced 
breathing we can greatly reduce the alveolar CO, 
percentage and also the quantity of CO, in the arte- 
rial blood. The effects of continued forced breathing 
are very marked. These are “swimming” of the head, 
abnormal sensations of “pins and needles,” loss of 
sensibility, contractions of various groups of muscles, 
and gradual loss of consciousness. By breathing dur- 
ing rest air containing 6 per cent or more of CO,, or 
a less percentage during exertion, we can observe the 
effects of undue excess of CO,—headache, giddiness, 
and often rapid loss of consciousness. Breathing is 
so regulated as to avoid these and other ill effects of 
excess or deficiency of CO,. In other words the main- 
tenance of breathing is but one manifestation of the 
co-ordinated bodily activities of which the outcome is 
the maintenance of bodily activity and structure as a 
whole. Breathing is a manifestation of life and there- 
fore possesses its characteristic features. 
It is evident that the mechanistic school of physi- 
ologists can point to the new facts with regard to the 
regulation of breathing as a confirmation of their 
principles. For the respiratory centre may be re- 
garded as a mechanism which reacts in a very sensi- 
tive manner to slight changes in the concentration of 
CO,. There is thus no mystery about the regulation 
of breathing—no need to invoke the presence of 
factors which are not physical or chemical. The 
respiratory centre is, in fact, typical of other bodily 
mechanisms. The delicacy of their reaction is due 
