RESPIRATION". 279 



below the rectum opens at the anus, and the bladder through the ure- 

 thra. All these openings, through which the hollow viscera communi- 

 cate with the exterior of the body, are guarded by muscles, called 

 sphincters, which can act independently of each other. 



Sighing. In sighing there is a somewhat 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 suddenly expelled. 



In the first, or inspiratory part of this act, the descent of the dia- 

 phragm presses the abdominal viscera downward, and of course this 

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

 with the exterior of the body. Inasmuch, however, as their various 

 openings are guarded by sphincters, in a state of constant tonic contrac- 

 tion, there is no escape of their contents, and the air simply enters the 

 lungs. In the second, or expiratory part of the act, pressure is also 

 made on the abdominal viscera in the opposite direction, by the 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, rectum, and 

 bladder, act in the same manner as before. 



Hiccough resembles sighing in that it is an inspiratory act: but the 

 inspiration is sudden instead of gradual, the diaphragm acting suddenly 

 and spasmodically; and the air, rushing through the unprepared rima 

 glottidis, is suddenly arrested and produces the peculiar sound. 



Coughing. In the act of coughing there is most often first of all a 

 deep inspiration, followed by an expiration; but the latter, instead of 

 being easy and uninterrupted, as in normal breathing, is obstructed, the 

 glottis being momentarily closed by the approximation of the vocal 

 cords. The abdominal muscles, then 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 noisily open the vocal cords which 

 oppose its outward passage. In this way considerable force is exercised, 

 and mucus or any other matter that may need expulsion from the air- 

 passages is quickly and sharply expelled by the outstreaming current of 

 air. It will be evident on reference to fig. 215, 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 

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

 contents as of the air in the lungs. The instinctive and if necessary 

 voluntarily increased contraction of the sphincters, however, prevents 

 any escape at the openings guarded by them, and the pressure is effec- 

 tive at one part only, at the rima glottidis. 



