CH. XXIV.] RESPIRATORY MUSCLES 353 



from one of these (head of rib) to the sternum (A B, fig. 327) ; the 

 motion of the rib around the latter axis being somewhat after the 

 fashion of raising the handle of a bucket. 



The elevation of the ribs is accompanied by a slight opening out 

 of the angle which the bony part forms with its cartilage ; and thus 

 an additional means is provided for increasing the antero-posterior 

 diameter of the chest. 



The muscles by which the ribs are raised, in ordinary quiet 

 inspiration, are the external *" intercostals, and that portion of the 

 internal intercostals which is situated between the costal cartilages ; 

 and these are assisted by the levatores costarum, and the serratus 

 posticus superior. 



In tranquil breathing, the expansive movements of the lower part 

 of the chest are greater than those of the upper. In forced inspira- 

 tion, on the other hand, the greatest extent of movement appears to 

 be in the upper antero-posterior diameter. 



In extraordinary or forced inspiration, as in violent exercise, or in 

 cases in which there is some interference with the due entrance of 

 air into the chest, and in which, therefore, strong efforts are necessary, 

 other muscles than those just enumerated are pressed into service. 

 It is impossible to separate by a hard-and-fast line the muscles of 

 ordinary from those of extraordinary inspiration ; but there is no 

 doubt that the following are but little used as respiratory agents, 

 except in cases in which unusual efforts are required the sterno- 

 mastoid, the serratus magnus, the pectorales, and the trapezius. Laryn- 

 geal and face muscles also come into play. 



The expansion of the chest in inspiration presents some peculi- 

 arities in different persons. In young children, it is effected chiefly 

 by the diaphragm, which being highly arched in expiration, becomes 

 flatter as it contracts, and, descending, presses on the abdominal 

 viscera, and pushes forward the front walls of the abdomen. The 

 movement of the abdominal walls being here more manifest than that 

 of any other part, it is usual to call this the abdominal type of respira- 

 tion. In men, together with the descent of the diaphragm, and the 

 pushing forward of the front wall of the abdomen, the chest and the 

 sternum are subject to a wide movement in inspiration (inferior costal 

 type). In women, the movement appears less extensive in the lower, 

 and more so in the upper, part of the chest (superior costal type). 



There are also differences in different animals. In the frog, for 

 example, the air is forced into the lungs by the raising of the floor of 

 the mouth, the mouth and nostrils being closed. 



Expiration. From the enlargement produced in inspiration, the 

 chest and lungs return, in ordinary tranquil expiration, by their 

 elasticity ; the force employed by the inspiratory muscles in distend- 

 ing the chest and overcoming the elastic resistance of the lungs and 



z 



