RESPIRATION. 



349 



transverse processes of the cervical vertebrae, and after pursuing a 

 downward and forward direction are inserted into the sternal end of 

 the first and second ribs. The action of the first two, at least, is to 

 elevate the first rib and thus establish a fixed point from which the 

 intercostal muscles can act. The posticus has doubtless a similar 

 action on the second rib. ^CV 



The levatores costarum are twelve in number on either side. They 

 arise from the tips of the transverse processes of the^ast cervical and 

 the thoracic vertebras with the exception of the last\\ From the point 

 of origin the fibers pass downward and outward in a diverging manner 

 to be inserted into the ribs between the tubercle and the angle & Their 

 action, as their name implies, is to elevate the posterior portion of 

 the ribs (Fig. 169). 



The serratus posticus superior, 

 a quadrilateral sheet of muscle- 

 fibers, arises mainly from the spines 

 of the last cervical and first and 

 second thoracic vertebrae. The an- 

 terior extremity is serrated and 

 attached to the outer surfaces of 

 the second, third, fourth, and fifth 

 ribs beyond the angle. The action 

 of the muscle is the elevation of the 

 ribs to which it is attached. 



The intercostales externi, eleven 

 in number on each side, occupy the 

 spaces between the ribs to which 

 they are attached from the tubercle 

 to the anterior extremity (Fig. 169). 

 Their fibers, which are arranged in 

 parallel bundles, are directed from 

 above downward and from behind 

 forward. The point of attachment, 



therefore, of any given bundle of fibers to the rib above lies nearer 

 the vertebral column, nearer the fulcrum, than the point of attach- 

 ment below. 



The action of the external intercostal muscles has been a subject 

 of much discussion;^ Some investigators have maintained that they 

 are elevators of the ri DS, and therefore inspiratory ; others that they are 

 depressors of the ribs, and therefore expiratory in function. At the 

 present time the general consensus of opinion is that the former view 

 is the one most in accordance with the facts.\v.The situation of the 

 muscles and the shortness of their fibers render it extremely diffi- 

 cult to obtain myographic tracings of their action. This is supposed, 

 however, to be disclosed by the play of the apparatus suggested 



FlG. 169. VIEW FROM BEHIND OF 

 FOUR DORSAL VERTEBRAE AND 

 THREE ATTACHED RIBS, SHOW- 

 ING THE ATTACHMENT OF THE 

 ELEVATOR MUSCLES OF THE RIBS 

 AND THE INTERCOSTALS. i. 

 Long and short elevators. 2. 

 External intercostal. 3. Internal 

 intercostal. (Allen Thomson.) 



