THE MECHANICS OF THE RESPIRATORY MOVEMENTS 



469 



correctness of this explanation has been questioned by A. v. Haller, who regarded 

 both intercostals as inspiratory in their function. As is indicated in Fig. 244, 

 two bars are arranged in such a way as to represent two adjoining ribs {A and B) 

 suspended from the vertebral column and united in front with the costal cartilages 

 a and b, and the sternum {S). The external intercostal and intercartilaginous 

 muscles are represented by rubber bands, placed in the position E and Ei, and 

 J and J I respectively. If, to begin with, the parallel bars are depressed sufficiently 

 to place the rubber bands under elastic tension (Fig. 244, /), their release is imme- 

 diately followed by an upward movement (Fig. 244, //) for the reason that the 

 elastic forces, although acting in opposite directions, are equal, and as they are 

 exerted in a parallelogram, the component acting upward on the long arm of the 

 lever exceeds the component acting downward on the short arm of the lever. 

 Moreover, as the distance between the consecutive ribs is fixed, their upward 

 movement must increase the angle between them and the costal cartilages, and 

 must also lead to a forward movement of the sternum. 



Fig. 244. — Diagram Illustrating the Action of the External Intercostal and 

 Intercartilaginei Muscles. 

 V, vertebral column; S, sternum; AB, two consecutive ribs; ab, their costal cartilages; 

 EEi, external intercostal ; JJi, intercartilaginei muscles; /, position at end of expiration, 

 when these muscles are under tension ; II, position at end of inspiration when these muscles 

 have contracted, raising the ribs and pushing the sternum forward. 



While the action of the external and internal intercostal muscles 

 has been a subject of much controversy, it is commonly held to-day 

 that the former elevate and the latter depress the ribs. For this 

 reason, the external one should be classified as an inspiratory, and the 

 internal one as an expiratory muscle. It is to be noted, however, that 

 they are not activated at the same time, because those placed between 

 the lower ribs serve as an aid to the diaphragm and are seldom at rest, 

 whereas those situated between the upper ribs remain inactive for 

 long periods of time until a fuller expansion of the lungs is required. 

 Besides, it should not be forgotten that the intercostal muscles serve 

 as tensors of the intercostal spaces. As Landois^ has shown, if the soft 

 parts between the ribs were perfectly flabby, they would be pulled 

 inward by the elastically recoiling lung, and more markedly so during 

 the inspiratory movements. Obviously, the alternate contraction 

 of these muscles prevents this bellying inward during practically the 

 1 Lehrbuch der Physiol., i, 1909, 190. 



