376 RESPIRATION. 



as in expiration, and distended with air, 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 con- 

 siderable 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 widen- 

 ed in inspiration. 



These points are clearly illustrated in the accompanying 

 diagrams. 1 



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 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 the sternal extremities are 

 movable, the direction of the fibres of the intercostals, from 

 above downwards and forwards, renders elevation of the ribs 

 a necessity of their contraction ; if it can be assumed that the 

 first rib is fixed, or at least does not move downwards. The 

 scalene muscles elevate the first rib in ordinary inspiration ; 

 and in deep inspiration, this takes place to such an extent as 

 to palpably carry with it the sternum and the lower ribs. 

 Theoretically, then, the external intercostals can do nothing 

 but render the ribs more nearly horizontal. The action of 

 these muscles has, however, been the subject of considerable 

 controversy, on theoretical grounds. We shall discuss the 

 question chiefly from an experimental point of view. 



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 cannot fail to convince any 

 one that these muscles enter into action in inspiration. This 



1 SIBSON, loc. cit 



