120 RESPIRATION RESPIRATORY MOVEMENTS. 



rest of the thorax. The articulation of the first rib with the vertebral column 

 is very movable, but it is joined to the sternum by a very short cartilage, 

 which allows of very little movement, so that its elevation necessarily carries 

 with it the sternum. This movement increases both the transverse and an- 

 tero-posterior diameters of the thorax, on account of the mode of articulation 

 and direction of the ribs, which are somewhat rotated as well as rendered 

 more horizontal. 



Intercostal Muscles. Concerning the mechanism of the action of these 

 muscles there is considerable difference of opinion among physiologists ; so 

 much, indeed, that the question is still left in some uncertainty. The most 

 extended researches on this point are those of Beau and Maissiat (1843), and 

 Sibson (1846). The latter seem to settle the question of the mode of action 

 of the intercostals and explain satisfactorily certain points which even now 

 are not generally appreciated. Onimus, and more recently, Laborde, have 

 shown, by experiments upon decapitated criminals, that the external inter- 

 costals raise and the internal intercostals depress the ribs, thus confirming 

 the views of Sibson. 



In the dorsal region, the spinal column forms an arch with its concavity 

 looking toward the chest, and the ribs increase in length progressively, from 

 above downward, to the deepest portion of the arch, where they are longest, 

 and then become progressively shorter. " During inspiration the ribs ap- 

 proach to or recede from each other according to the part of the arch with 

 which they articulate ; the four superior ribs approach each other anteriorly 

 and recede from each other posteriorly ; the fourth and fifth ribs, and the 

 intermediate set (sixth, seventh, and eighth), move further apart to a mod- 

 erate, the diaphragmatic set (four inferior), to a great extent. The upper 

 edge of each of these ribs glides toward the vertebrae in relation to the lower 



edge of the rib above, with the exception of the 

 lowest rib which is stationary" (Sibson). These 

 movements increase the antero-posterior and trans- 

 verse diameters of the thorax. As the ribs are ele- 

 vated and become more nearly horizontal, they 

 must push forward the lower portion of the ster- 

 num. Their configuration and mode of articula- 

 tion with the vertebra? are such that they can not 

 be elevated without undergoing a considerable ro- 

 tation, by which the concavity looking directly to- 

 ward the lungs is increased, and with it the lateral 

 diameter of the chest. All the intercostal spaces 

 posteriorly are widened in inspiration. 

 inspiration <B6ciard). The ribs are elevated by the action of the ex- 



The dark lines represent the ribs, , . , . . A , . , 



sternum and costal cartilages ternal intercostals, the sternal portion of the inter- 

 nal intercostals and the levatores costarum. The 



external intercostals are situated between the ribs only, and are wanting in 

 the region of the costal cartilages. As the vertebral extremities of the ribs 

 are the pivots on which these levers move, and as the sternal extremities are 



