RESPIRATION. 199 



on through the mouth and nostrils through either of them separately, 

 or through both at the same time, according to the position of the soft 

 palate. The stomach communicates with the exterior of the body through 

 the oesophagus, pharynx, and mouth; while below the rectum opens at 

 the anus, and the bladder through the urethra. All these openings, 

 through which the hollow viscera communicate with the exterior of the 

 body, are guarded by muscles, called sphincters, which can act independ- 

 ently of each other. The position of the latter is indicated in the dia- 

 gram. 



FIG. 161. 



Sighing. In sighing there is a rather prolonged inspiration; the air 

 almost noiselessly passing in through the glottis, and by the elastic recoil 

 of the lungs and chest- walls, and probably also of the abdominal walls, 

 being rather suddenly expelled again. 



Now, in the first, or inspiratory part of this act, the descent of the 

 diaphragm presses the abdominal viscera downward, and of course this 

 pressure tends to evacuate the contents of such as communicate with the 

 exterior of the body. Inasmuch, however, as their various openings are 

 guarded by sphincter muscles, in a state of constant tonic contraction, 



