THE MECHANICS OF THE RESPIRATORY MOVEMENTS 465 



yt 



possible expansion of the lungs, points toward a certain difference in 

 the structure or constitution of the diaphragmatic myoplasm. 



The increase in the anteroposterior and transverse diameters of the 

 chest is effected in the following manner: Posteriorly, the heads of 

 the different ribs are in articulation with the vertebral column forming 

 here partially movable joints. Anteriorly, on the other hand, the first 

 ten pairs of ribs are anchored to the sternum by the costal cartilages, 

 the upper seven of them being connected with this bone directly and 

 the lower three indirectly, while the eleventh and . twelfth pairs of 

 ribs remain free and functionally constitute a part of the abdominal 

 wall. When at rest, the different ribs in- 

 cline obliquely downward and forward, so 

 that their anterior extremities come to lie at 

 a lower level than their posterior. During in- 

 spiration they are elevated and rotated out- 

 ward, this movement being made possible by 

 the flexibility of the costal cartilages and the 

 yielding character of the costochondral and 

 chondrosternal articulations. It is evident 

 that the lessening of the obliquity of the ribs 

 increases the distance between the sternum 

 and the spinal column; moreover, inasmuch 

 as the different pairs of ribs form rings, the 

 diameters of which steadily increase from 

 above downward until the seventh pair has 

 been reached, it necessarily follows that their 

 elevation also increases the breadth of the 

 thorax. Thus, the seventh rib is raised to the 

 level previously occupied by the sixth, and the 

 sixth to that of the fifth, and so on until the 

 first has been reached. In addition, it is to 

 be noted that the elevation of the ribs is as- 

 sociated with a slight outward rotation at 

 their angles. This may be gathered from the 



fact that their external surfaces are turned outward and downward 

 on expiration, and directly outward on inspiration. This rotation 

 alone is sufficient to increase the transverse diameter of the chest. 



Each rib articulates with the spinal column in two places. Its 

 head lies in contact with the body of the vertebra and its tubercle 

 with the transverse process. In moving upward the different ribs 

 rotate around an axis drawn through these two points, but inasmuch 

 as their shafts are directed obliquely downward on expiration, their ele- 

 vation during inspiration forces their sternal ends farther outward and 

 away from the spinal column. Although this movement is greatly re- 

 stricted, because the ribs are not freely movable upon the sternum, 

 these articulations are rendered more yielding by the interposition 



30 



FIG. 240. DIAGRAM TO 

 ILLUSTRATE THE EFFECT OF 

 THE SLANT OF THE RIBS. 



S, the spinal column; a, 

 the position of the rib in 

 normal expiration; a' its 

 position (exaggerated) in 

 inspiration. The distance 

 between the spinal column 

 and the sternum (st.), i.e., 

 the anteroposterior diam- 

 eter of the chest is in- 

 creased). (American Text- 

 book of Physiology.) 



