INSPIRATION 



315 



tances of the sternal ends of the different pairs of ribs from the spinal column 

 are unequal, the separate segments of the sternum must be moved unequally 

 and must be bent on each other; and, what is more important,, the costal 

 cartilages are thrown into a twist. Naturally this occasions some resistance 

 to the elevation of the ribs, which, in addition to the resistance of their weight 

 and of the negative pressure in the thoracic cavity, must be overcome by the 

 muscles of inspiration. 



If the ribs be moved out of their natural position by any force and this 

 force then cease to act, they will return of themselves to the position of rest 

 by reason of the above-mentioned anatomical circumstances. 



Since, therefore, the elevation of the ribs causes an expansion of the chest, we 

 shall designate as inspiratory all those 

 muscles by the contraction of which 

 the ribs are raised. This is not equiva- 

 lent to saying that these muscles al- 

 ways act in inspiration. Some of the 

 rib-lifting muscles, be it expressly ob- 

 served, are active only in very excep- 

 tional cases, while in natural, quiet 

 breathing only certain of the muscles 

 participate. 



The most widely different views 

 have been expressed from time to time 

 as to what muscles actually lift the ribs. 

 This is especially true of the intercostal 

 muscles. In the opinion of some, both 

 the external and the internal intercostals 

 are inspiratory muscles, in the opinion 

 of others both are muscles of expiration ; 

 others again believe that the external 

 tend to raise, the internal to depress the 

 ribs; and finally, the view has been 

 maintained that these muscles are pres- 

 ent only for the purpose of regulating 

 the tension in the intercostal spaces 

 and of rotating the thorax in its long 

 axis. By observations on living animals 

 in which all the muscles of respiration 

 were excluded except the intercostals, 

 it has now been made clear that the 



outer layer, as well as that part of the inner included between the costal carti- 

 lages, serves to elevate the ribs, while the remainder of the inner layer draws 

 the ribs down (Bergendal and Bergman, R. Du Bois-Reymond and Masoin, R. 

 Pick). 



In the rabbit at least the intercostal muscles are the most important so far 

 as the thoracic breathing is concerned. When greater demand is made upon the 

 muscles of inspiration, the levatores cosiarurti and the scaleni are added first. 

 The levatores alone are able to look after the respiratory movements for a cer- 

 tain time, and their action in the cat is very important (Koraen and B. Moller). 

 Since these muscles are inserted quite close down to the hinder ends of the ribs, 



FIG. 124. The thorax seen from the right 

 side, after Spalteholtz. 



