INTEECOSTAL MUSCLES. 377 



fact has been observed by Sibson and many other physiolo- 

 gists. If attention be now directed to the sternal portion of 

 the internal intercostals, situated between the costal cartilages, 

 their fibres having a direction from above downwards and 

 backwards, it is equally evident that they enter into action 

 with inspiration. By artificially inflating the lungs after 

 death, Sibson confirmed these observations, and showed that 

 when the lungs are filled with air, the fibres of these muscles 

 are shortened. In inspiration the ribs are all separated pos- 

 teriorly ; but laterally and anteriorly, some are separated (all 

 below the fourth), and some are approximated (all above the 

 fourth). Thus all the interspaces, excepting the anterior por- 

 tion of the upper three, are widened in inspiration. Sibson 

 lias shown by inflation of the chest, that though the ribs are 

 separated from each other, the attachments of the intercostals 

 are approximated. The ribs, from an excessively oblique 

 position, are rendered nearly horizontal; and consequently 

 the inferior attachments of the intercostals are brought nearer 

 the spinal column, while the superior attachments on the 

 upper borders of the ribs are slightly removed from it. Thus 

 these muscles are shortened. If, by separating and elevating 

 the ribs, the muscles are shortened, shortening of the muscles 

 will elevate and separate the ribs. In the three superior 

 interspaces, the constant direction of the ribs is nearly hori- 

 zontal, and the course of the intercostal fibres is not as oblique 

 as in those situated between the lower ribs. These spaces are 

 narrowed in inspiration. The muscles between the costal 

 cartilages have a direction opposite to that of the external 

 intercostals, and act upon the ribs from the sternum, as the 

 others do from the spinal column. The superior interspace 

 is narrowed, and the remainder are widened, in inspiration. 



The probable explanation of the great difference of opin- 

 ion with regard to the action of the intercostals is the diffi- 

 culty of comprehending, at the first blush, that the contrac- 

 tion of muscles situated between the ribs can separate them 

 from each other ; a phenomenon which is easily understood 



