THE MECHANICS OF PULMONARY RESPIRATION. 341 



toward the position of the one above it, the chest at that level will become 

 wider from side to side, in proportion as the fifth arch is wider than the 

 fourth. Thus the elevation of the rib increases not only the antero-posterior 

 but also the transverse diameter of the chest. Further, on account of the 

 resistance of the sternum, the angles between the ribs and their cartilages 

 are, in the elevation of the ribs, somewhat opened out, and thus also the 

 transverse as well as the antero-posterior diameter, somewhat increased. In 

 more than one way, then, the elevation of the ribs enlarges the dimensions 

 of the chest. 



276. The ribs are raised by the contraction of certain muscles. Of 

 these the external intercostals are perhaps the most important. Even in the 

 case where two ribs, such as the fifth and sixth, are isolated from the rest of 

 the thoracic cage, by section of the structures occupying the intercostal spaces 

 above and below, the contraction of the external intercostal muscle of the 

 intervening space raises the two ribs, thus bringing them toward the posi- 

 tion in which the fibres of the muscle have the shortest length, viz., the hori- 

 zontal one. This elevating action is, in the entire chest, further favored by 

 the fact that the first rib is less movable than the second, and so affords a 

 comparatively fixed base for the action of the muscles between the two, the 

 second in turn supporting the third, and so on, while the scaleni muscles in 

 addition serve to render fixed, or to raise, the first two ribs. So that in 

 normal respiration, the act may probably be described as beginning by a 

 contraction of the scaleni. The first two ribs being thus raised or at least 

 fixed, the contraction of the series of external intercostal muscles acts at a 

 great disadvantage. 



While the elevating, i. e., inspiratory, action of the external intercostals 

 is admitted by nearly all authors, the function of the internal intercostals has 

 been much disputed. Some regard their action as wholly inspiratory ; others 

 maintain, what is perhaps the more commonly adopted view, that while those 

 parts of them which lie between the sternal cartilages act like the external 

 intercostals as elevators, i. e., as inspiratory in function, those parts which 

 lie between the osseous ribs act as depressors, i. e., as expiratory in func- 

 tion. 



In the well-known model consisting of two rigid bars representing the 

 ribs, moving vertically by means of their articulations within an upright 

 representing the spine, and connected at their free ends by a piece repre- 

 senting the sternum, it is undoubtedly true that stretched elastic bands 

 attached to the bars in such a way as to represent respectively the external 

 and internal intercostals, viz., sloping in the one case downward and forward, 

 and in the other downward and backward, do, on being left free to contract, 

 in the former case elevate and in the latter depress the ribs. Such a model, 

 however, does not fairly represent the natural conditions of the ribs, which 

 are not straight and rigid, but peculiarly curved and of varying elasticity, 

 capable moreover of rotation on their own axes, and having their move- 

 ments determined by the characters of their vertebral articulations. The 

 mechanical conditions in fact of these muscles are so complex, that a deduc- 

 tion of their actions from simple mechanical principles, or from the direc- 

 tion of the fibres, must be exceedingly difficult and dangerous. Actual ex- 

 periments on the cat and dog tend to show that in these animals the contrac- 

 tion of the internal intercostals, along their whole length, takes place, in 

 point of time, alternately with that of the diaphragm, and thus offer an 

 argument in favor of these muscles being expiratory in function. 



Next in importance to the external intercostals come the levatores 

 costarum, which, though small muscles, are able, from the nearness of their 

 costal insertions to the fulcrum, to produce considerable movement of the 



