2 3 2 RESPIRATION. 



separately, or through, both at the same time, according 

 to the position of the soft palate. The stomach commu- 

 nicates with the exterior of the body through the oesopha- 

 gus, 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 commu- 

 nicate with the exterior of the body are guarded by 

 muscles, called sphincters, which can act independently 

 of each other. The position of the latter is indicated in 

 the diagram. 



Let us take first the simple act of sighing. In this case 

 there is a rather prolonged inspiratory effort by the dia- 

 phragm and other muscles concerned in 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 sud- 

 denly expelled again. 



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 oi 

 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 expira- 

 tory 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 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 



