182 RESPIRATION. 



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

 of the diaphragm presses the abdominal viscera downwards, 

 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, there is no 

 escape of their contents, and air simply enters the lungs. In 

 the second, or expiratory part of the act of sighing, there is also 

 pressure made on the abdominal viscera in the opposite direc- 

 tion, by the elastic or muscular recoil of the abdominal walls ; 

 but the pressure is relieved by the escape of air through the 

 open glottis, and the relaxed diaphragm is pushed up again 

 into its original position. The sphincters of the stomach, rec- 

 tum, and bladder act as before. 



Hiccough resembles sighing in that it is an inspiratory act, 

 but the inspiration is sudden instead of gradual, from the 

 diaphragm acting suddenly and spasmodically ; and the air, 

 therefore, suddenly rushing through the unprepared rima 

 glottidis, causes vibration of the vocal cords, and the peculiar 

 sound. 



In the act of coughing, there is most often first an inspira- 

 tion, and this is followed by an expiration ; but when the lungs 

 have been filled by the preliminary inspiration, instead of the 

 air being easily let out again through the glottis, the latter is 

 momentarily closed by the approximation of the vocal cords ; 

 and then the abdominal muscles, strongly acting, push up the 

 viscera against the diaphragm, and thus make pressure on the 

 air in the lungs until its tension is sufficient to burst open 

 noisily the vocal cords which oppose its outward passage. In 

 this way a considerable force is exercised, and mucus or any 

 other matter that may need expulsion from the lungs or trachea 

 is quickly and sharply expelled by the out-streaming current 

 of air. 



Now it is evident on reference to the diagram (Fig. 65), 

 that pressure exercised by the abdominal muscles in the act of 

 coughing, acts as forcibly on the abdominal viscera as on the 

 lungs, inasmuch as the viscera form the medium by which the 

 upward pressure on the diaphragm is made, and of necessity 

 there is quite as great a tendency to the expulsion of their con- 

 tents as of the air in the lungs. The instinctive and, if neces- 

 sary, voluntarily increased contraction of the sphincters, how- 

 ever, prevents any escape at the openings guarded by them, 

 and the pressure is effective at one part only, namely, the rima 

 glottidis. 



The same remarks that apply to coughing, are almost ex- 

 actly applicable to the act of sneezing ; but in this instance 



