103 Dr. W. Marcet. 



muscles attended with fatigue and with an expiration of carbonic 

 acid increased beyond the normal. This, however, is due mainly to 

 the displacement of carbonic acid present normally in the blood, a 

 small proportion of C0 2 only being the result of the work done by 

 the action of the muscles concerned in the forced respiration. A 

 sadden return from forced to normal breathing in repose is attended 

 with an involuntary double reaction, the first being a respiratory 

 pause, and the second a slight increase of the respiration more or 

 less marked according to the intensity of the first reaction and 

 merging into natural breathing. 



3. Respiration under exercise, exhibiting characters shown in the 

 tracings, and in other respects described in a previous paper. 



4. Breathing in a state of muscular repose ivliile under the influence 

 of a strong effort of volition. The characters of this form of respira- 

 tion partake of breathing under muscular exercise and of forced breath- 

 ing. Like breathing during exercise, the volume of air respired is 

 increased, and the action of the respiratory muscles is automatic and 

 unattended with any fatigue, while there is an elimination of an 

 increase of CO 2 . Like forced breathing, on releasing the influence 

 of the will there is a slight reaction or pause, but it is distinctly less 

 marked than after forced breathing, and the following after stage 

 observed in forced breathing, is barely, if at all perceptible. From 

 these circumstances it is concluded, though as it were from circum- 

 stantial evidence, that volition strongly exerted is productive of the 

 formation of carbonic acid. 



The second form of breathing, or forced expiration, includes sneez- 

 ing, sighing, and yawning. 



The third form of breathing, or under exercise, includes what may 

 be called vocal breathing, or reading, talking, singing, weeping, 

 laughing, shouting, and coughing. In these instances there is but 

 little or no CO 2 absorbed in the blood as it is given out as the exercise 

 proceeds. 



The fourth form of breathing may be considered as including every 

 kind of mental exertion. 



Finally, the different forms of breathing may assist or clash with 

 each other. Thus forced breathing assists breathing under exercise, 

 especially when repose follows exercise. 



Forced respiration may clash with vocal respiration a<s shown by 

 the difficulty experienced in talking or singing immediately after 

 forced breathing. 



Appendix. 



An explanation is wanted with reference to the production of heat 

 required for the labour of forced breathing. It might be objected in 

 accordance with the statement of Him, in his book on the " Mechani- 



