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TEXT-BOOK OF PHYSIOLOGY. 



The action of the external intercostal muscles, as well as the action of 

 the inter car tilaginei muscles, has been a subject of much discussion. Some 

 investigators have maintained that they are elevators of the ribs, and there- 

 fore 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. In 

 the following explanation as to their action, their relation to the ribs and to 

 the cartilages, must be recalled to mind. The relation of the external inter- 

 costals is such that the point of attachment of any given bundle of fibers 

 to the rib above lies nearer the vertebral column, nearer the fulcrum, 

 than the point of attachment to the rib below. The relation of the inter- 

 cartilaginei to the cartilages is such that the point of attachment of any 

 given bundle of fibers to the cartilage above lies nearer the sternum, nearer 

 the fulcrum, than the point of attachment to the cartilage below. The 

 situation of the muscles and the shortness of their fibers render it extremely 

 difficult to obtain myographic tracings which would elucidate their action 

 in elevating the ribs and cartilages. 



A clear conception of their action, however, may be arrived at by the 

 study of the schematic model first presented by Hamberger. Fig. 191. In 



FIG. 191. DIAGRAMS ILLUSTRATING THE ACTION OF THE EXTERNAL INTERCOSTAL AND IN- 



TERCARTILAGINEI MUSCLES. 



this model v-v r is a vertical support carrying two freely movable parallel 

 bars rr', united at their opposite ends with two other freely movable 

 and parallel bars cc, carried by a second vertical supports, representing 

 respectively the vertebral column, two adjoining ribs, two adjacent cartilages, 

 and the sternum. Diagram A shows the position of the different parts at 

 the end of expiration and B their position at the end of inspiration. The 

 parallel bars are joined to each other by elastic bands ei and ic having the 

 direction of and representing the external intercostal and intercartilaginei 

 muscles, respectively. The bars are depressed to sufficiently elongate and 

 tense the elastic bands thus and imitate the condition of the muscles in so 

 far as tension is concerned prior to their contraction. On releasing the 

 bars the elastic bands at once recoil and the bars representing ribs and 

 cartilages are raised. Although the elastic forces, acting in opposite direc- 

 tions as indicated by the arrows are equal, the bars are yet raised for the 

 reason that in accordance with the parallelogram of forces, the component 



