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 



