CH. XXIV.] MECHANISM OF RESPIRATION. 349 



a line drawn from one of these (head of rib) to the sternum 

 (A B, fig. 318) ; 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 wich its cartilage 

 (fig. 319, A); 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. The action of the levatores and the serratus is 

 very simple. Their fibres, arising from the spine as a fixed point, 

 pass obliquely downwards and forwards to the ribs, and neces- 

 sarily raise the latter when they contract. The action of the 

 intercostal muscles is not quite so simple, inasmuch as, passing 

 merely from rib to rib, they seem at first sight to have no fixed 

 point towards which they can pull the bones to which they are 

 attached. 



In tranquil breathing, the expansive movements of the lower 

 part of the chest are greater than those of the upper. In forced 

 inspiration, 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 sternomastoid, the serratus magnus, the pectorales, 

 and the trapezius. Laryngeal and face muscles also come into play. 



The expansion of the chest in inspiration presents some 

 peculiarities in different persons. In young children, it is effected 

 chiefly by the diaphragm, which being highly arched in expira- 

 tion, 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 respiration. 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, 



