220 PHYSIOLOGY FOR DENTAL STUDENTS. 



that the collapse of the alveoli of the lungs which occurs at the 

 end of normal expiration, and the stretching of the alveolar walls 

 which occurs at the end of normal inspiration, cause stimuli to be 

 passed along the vagi to the center, and that these stimuli bring 

 on the next phase of respiration. The breaking of the connection 

 between the lungs and the alveoli destroys this influence and the 

 respirations become deep and slow. 



In the absence of the vagi, the higher centers assume partial 

 control of the regulation of the respiratory movements. If they 

 also are destroyed, however, breathing becomes inadequate to 

 maintain life, although the center itself is still able to keep up a 

 modified, rhythmic respiration. 



REFLEX RESPIRATORY MOVEMENTS. The cutaneous nerves, es- 

 pecially those of the face and abdomen, have a marked influence 

 on respiration. These can be excited by heat or cold or pain; 

 for instance, a cold bath will cause a deepening or quickening of 

 the respiration. An example is found in the forced expiratory 

 effort made on inhalation of acid or sharp smelling substances, 

 which not only affect the olfactory nerves, but also the sensitive 

 endings of the fifth nerve in the nasal mucous membra in-. 



Chemical Control of Respiration.-^In spite of this very effec- 

 tive method of nervous control of the respiration, there is an- 

 other no less important means of respiratory control, which de- 

 pends on the ability of chemical substances in the blood to stim- 

 ulate the respiratory center. The substances which most readily 

 affect the center are acids, such as carbon dioxide (which in solu- 

 tion forms a weak acid,) and lactic acid, which is formed under 

 certain conditions in the body. Lack of oxygen, if it be consid- 

 erable, also causes the center to show marked signs of activity. 

 In the introductory chapter the physico chemical properties of 

 the blood and tissue fluids were discussed. It will be recalled 

 that these are practically neutral fluids, that is, they show an al- 

 most exact balance in the number of hydrogen and hydroxyl 

 ions, a condition which determines the neutrality of a fluid. Any 

 increase in the amount of carbon dioxide in the blood would form 

 proportionately more carbonic acid, which yields hydrogen ions, 

 and thus tend to destroy the neutral balance of the blood. This 



