I 
THE REGULATION OF BREATHING 
Animal physiology deals with the activities ob- 
served in living animals, including men; but under 
certain limitations. It deals in the first place with 
all the activities which are unconscious, such as diges- 
tion, circulation of the blood, secretion, or the growth 
and maintenance of the tissues. It deals, also, with 
the unconscious element in conscious action. I may, 
for instance, breathe consciously, or move my pen in 
writing, or hear the noise which it makes; but of the 
details involved in any of these acts I have no direct 
consciousness. They are only revealed by experi- 
mental physiology. Physiology deals, also, with the 
sensations, impulses, and instincts of all kinds which 
appear in consciousness; but does not deal with the 
meaning and conscious control which are attached to 
them. It does not deal with this meaning and con- 
scious control for the very good reason that the facts 
relating to them cannot be combined with the other 
material of physiology into a homogeneous sys- 
tem of scientific knowledge. If, however, the mean- 
ing and conscious control attached to sensation and 
instinct are disregarded, the latter can be treated as a 
part of physiology, and are so treated by physiologists. 
When the activities included as physiological are 
