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ANATOMY AND PHYSIOLOGY 



piration, the intercostals having considerable power, but the dia- 

 phragm being most important. When it contracts it is depressed, 

 increasing the depth of the thoracic cavity, while the other muscles 

 broaden the cavity by lifting the ribs, and thus room is made for 

 expansion of the lungs in inspiration. As the ribs fall and the 

 diaphragm ceases to contract, it rises, returning to its dome shape, 

 and thus the air is pressed from the lungs in expiration. These 



FIG. 234. THORACIC AND ABDOMINAL VISCERA, ANTERIOR. (Deaver.) 



two acts complete a respiration, or an act of breathing, which 

 occurs normally about eighteen times in a minute. If respiration 

 is very difficult other muscles are called into play, as in asthma, 

 when the struggle for breath is so great that " forced inspiration" is 

 necessary. 



The erector spinae muscles are always on duty, to steady the 

 spine in order that the ribs may have a point of departure. 



The cardiac impulse is felt (sometimes it may be seen) between 



