382 COMPARATIVE PHYSIOLOGY. 



respiratory tract. To these sounds names have been given, hut 

 as they are somewhat inconstant in their application, or at least 

 have several synonyms, we pass them by, recommending the 

 student to actually learn the nature of the respiratory murmurs 

 by listening to the normal chest in both man and the lower ani- 

 mals. With the use of a double stethoscope he may practice 

 upon himself, though not so advantageously as in the case of 

 the heart. 



The sounds are caused iii part by the friction of the air, 

 though they are probably complex, several factors entering 

 into their causation. 



COMPAIUSON OF THE INSPIRED AND EXPIRED AIR. 



The changes that take place in the air respired may be brief- 

 ly stated as follows : 



1. Whatever the condition of the inspired air, that expired 

 is about saturated with aqueous vapor — i. e., it contains all that 

 it is capable of holding at the existing temperature. 



2. The temperature of the expired air is about that of the 

 blood itself, so that if the air is very cold when breathed, the 

 body loses a great deal of its heat in warming it. The expired 

 air of the nasal passages is slightly warmer than that of the 

 mouth. 



3. Experiment shows that the expired air is really dimin- 

 ished in volume to the extent of from one fortieth to one fif- 

 tieth of the whole. Since two volumes of carbonic anhydride 

 require for their composition two volumes of oxygen, if the 

 amount of the former gas expired be not equal to the amount 

 of oxygen inspired, some of the latter must have been used to 



form other combinations. -^, amounting to rather less than 



1, is called the respiratory coefficient. 



4. The di£ference between inspired and expired air in man 

 may be gathered from the following : 



Inspired air. 

 Expired air. 



From which the most important conclusions to be drawn 

 are, that the expired air is poorer in oxygen to the extent of 4 

 to 5 per cent, and richer in carbonic anhydride to somewhat 



