126 RESPIRATION. 



a late elaborate work assumes that the question is still left in considerable uncertainty. 

 The most extended researches on this point are those of Beau and Maissiat (Archives 

 generates de medecine, 1843), and Sibson (Philosophical Transactions, 1846). The latter 

 seern to settle the question of the mode of action of the intercostals and explain satis- 

 factorily certain points which even now are not generally appreciated. More recently, 

 Onimus has shown, by experiments upon a decapitated animal, that the external inter- 

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

 Sibson. 



We shall first note the changes which take place in the direction of the ribs and their 

 relation to each other in inspiration, before considering the way in which these move- 

 ments are produced. 



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

 chest, and the ribs increase in length progressively, from above downward, to the deep- 

 est portion of the arch, where they are longest and then become progressively shorter. 

 According to Sibson, " during inspiration the ribs approach to or recede from each other 

 according to the part of the arch with which they articulate ; the four superior ribs ap- 

 proach 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 

 moderate, the diaphragmatic set (four inferior), to a great extent. The upper edge of 

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

 above, with the exception of the lowest rib, which is stationary." These movements 

 increase the antero-posterior and transverse diameters of the thorax. As the ribs are 

 elevated and become more nearly horizontal, they must push forward the lower portion 

 of the sternum. Their configuration and mode of articulation with the vertebrae are 

 such, that they cannot be elevated without undergoing a considerable rotation, by which 

 the concavity looking directly toward the lungs is increased, and with it the lateral 



diameter of the chest. All the intercostal spaces posteriorly 

 are widened in inspiration. 



The ribs are elevated by the action of the external inter- 

 costals, the sternal portion of the internal intercostals, and 

 the levatores costarum. The external intercostals are situ- 

 ated 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 movable, the direction of the fibres 

 of the intercostals from above downward and forward 

 renders elevation of the ribs a necessity of their contrac- 

 tion, if it can be assumed that the first rib is fixed or at 

 least does not move downward. The scalene muscles ele- 

 vate the first rib in ordinary inspiration; and, in deep in- 



. 4Z.-ElwaUon of the ribs in spiration, this takes place to such an extent as to palpably 

 inspiration. (Bceiard ) carrv with it the sternum and the lower ribs. Theoreti- 



Ine dark lines represent the ribs, * . , , . , 



sternum, and costal cartilages in cally, then, the external intercostals can do nothing but 

 render the ribs more nearly horizontal. 



If the external intercostals be exposed in a living animal, the dog, for example, in 

 which the costal type of respiration is very marked, close observation can hardly fail to 

 convince any one that these muscles enter into action in inspiration. This fact has 

 been observed by Sibson and many other physiologists. If attention be directed to the 

 sternal portion of the internal intercostals, situated between the costal cartilages, their 

 fibres having a direction from above downward and backward, 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 



