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HANDBOOK OF PHYSIOLOGY. 



in the lateral and antero-posterior diameters. The muscles engaged in 

 ordinary inspiration are the diaphragm; the scaleni; the external inter- 

 costals; parts of the internal intercostals; the levatores costarum ; and 

 serratus posticus superior. 



(a.) The vertical diameter of the chest is increased by the contraction 

 and consequent descent of the diaphragm the sides of the muscle de- 

 scending most, but the central tendon also- descends to some extent, 

 while the intercostal and other muscles, by acting at the same time, pre- 

 vent the diaphragm, during its contraction, from drawing in the sides 

 of the chest. 



#. The increase in the lateral and antero-posterior diameters of the 



Fig. 210. Diagram of axes of movement of ribs. 



chest is effected by the raising of the ribs, the greater number of which 

 are attached very obliquely to the spine and sternum. 



The elevation of the ribs takes place both in front and at the sides 

 the hinder ends being prevented from performing any upward move- 

 ment by their attachment to the spine. The movement of the front 

 extremities of the ribs is of necessity accompanied by an upward and 

 forward movement of the sternum to which they are attached, the move- 

 ment being greater at the lower end than at the upper end of the latter 

 bone. 



The axes of rotation in these movements are two; one corresponding 

 with a line drawn through the two articulations which the rib forms 

 with the spine (a, b, fig. 210) ; and the other with a line drawn from one 

 of these (head of rib) to the sternum (A B, fig. 210) ; the motion of the 

 rib around the latter axis being somewhat after the fashion of raising 

 the handle of a bucket. 



