RESPIRATION 



403 



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. 187. In 

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

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

 and parallel bars cc, cc' carried by a second vertical support, s, 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 and thus imitate the condition of the muscles in so 

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



A 



B 



FIG. 187. DIAGRAMS ILLUSTRATING THE ACTION OF THE EXTERNAL INTERCOSTAL AND IN- 

 TERCARTILAGINEI MUSCLES. 



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 components 

 acting upward on the long arms of the levers preponderate over the com- 

 ponents acting downward on the short arms of the levers. This, taken in con- 

 nection with the fact that the distances between the adjoining bars are fixed, 

 leads not only to an elevation of the bars, but to a widening of the angle 

 between them and an advance of the second vertical support. The actions 

 of these bands thus disclose and illustrate the actions of both the external 

 intercostal and intercartilaginei muscles. It must therefore be concluded 

 that these muscles are the elevators of the ribs and cartilages and hence, 

 inspiratory in function. Of late the correctness of Hamberger' s view has 

 been confirmed by experiments on living animals. 



The levatores costarum, as is evident from their points of origin and in- 

 sertion, elevate the ribs posteriorly. 



