RESPIRATION. 195 



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 inspirator^ 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 direction, 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, 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, in consequence of the diaphragm 

 acting suddenly and spasmodically ; and the air, therefore suddenly rush- 

 ing through the unprepared rima glottidis, causes vibration of the vocal 

 cords, and 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, in 

 consequence of the glottis being momentarily closed by the approxima- 

 tion 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 burst 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 expul- 

 sion from the air-passages is quickly and sharply expelled by the out- 

 streaming current of air. 



It will be evident on reference to the diagram (Fig. 160), that pres- 

 sure 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 effective at one part only, at the rima glottidis. 



Sneezing. The same remarks that apply to coughing, are almost 

 exactly applicable to the act of sneezing ; but in this instance the blast 

 of air, on escaping from the lungs, is directed, by an instinctive contrac- 



