RESPIRATION. 



63 



Fig. 14.* 



the dilatation of the chest during in- 

 spiration. In its relaxed state this 

 muscle is arched; by contracting it 

 becomes more plane; and by this 

 flattening of its arch, the capacity of 

 the thorax is increased, at the same 

 time that the abdominal viscera are 

 pressed upon from above, so as to 

 produce the protrusion of the abdo- 

 men observed during inspiration. 

 (Fig. 14.) 



In a natural, tranquil inspiration, 

 the dilatation of the chest is effected 

 almost entirely by the diaphragm. 

 The lateral dilatation, as in deep in- 

 spiration, is performed principally 

 by the action of the intercostal 

 muscles, assisted also by the sca- 

 leni, the levatores costarum, the 

 serratus posticus inferior, and the 

 thoracic muscles generally. In the 

 old, inspiration is mainly accom- 

 plished by the diaphragm, in conse- 

 quence of the ossification of the car- 

 tilages of the ribs. 



The number of inspiratory movements vary greatly under diffe- 

 rent circumstances. In general it may be stated from 14 to 18 

 usually occur in a minute ; of these the ordinary inspiration involves 

 but little movement of the thorax ; but a greater exertion is made at 

 about every fifth recurrence. 



The average numerical proportion of the respiratory movements 

 to the pulsations of the heart, is about 1 to 5 or 41, and when this 

 proportion is widely departed from, there is reason to suspect some 

 disorder of the respiratory apparatus or the nervous system. 



Expiration^ when perfectly tranquil, may be the result of the 

 mere collapse or elastic reaction of the parts recovering their na- 

 tural state, after the active dilatation they have undergone ; in fact, 

 tranquil respiration seems to consist more in the periodic action of 

 the muscles of inspiration, than in the alternate action of antago- 

 nising muscles. Among the elastic components of the respiratory 

 organs may be mentioned the cartilages of the ribs, the cartilages of 

 the bronchial tubes, and the capsule of elastic tissue described by M. 

 Bazin, which invests the lungs, and sends prolongations inwards, and 

 the elasticity of the air-cells themselves. The muscles which are 

 concerned in expiration are the abdominal, which draw down the 



* Section of thorax and abdomen. 

 d. Contracted diaphragm. 



a. Thorax, b. Abdomen, c. Relaxed diaphragm. 



