100 RESPIRATION 



the mode of action of the intercostals and explain satisfactorily certain 

 points which even now are not commonly appreciated. Onimus and, 

 more recently, Laborde have shown by experiments on decapitated 

 criminals that the external intercostals raise and the internal intercostals 

 depress the ribs, confirming the views of Sibson. 



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

 cavity looking toward the chest, and the ribs increase in length progres- 

 sively, from above downward, to the deepest portion of the arch, where 

 they are longest, and then progressively become shorter. " During in- 

 spiration the ribs approach 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 farther apart to a moderate, 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 transverse diameters of the 

 thorax. As the ribs are elevated and become more nearly horizontal, 

 they push forward the lower portion of the sternum. Their configuration 

 and mode of articulation with the vertebrae are such that they can not be 

 elevated without undergoing a considerable rotation, by which the con- 

 cavity looking directly toward the lungs is increased, and with it the 

 bilateral diameter of the chest. All the intercostal spaces posteriorly 

 are widened in inspiration. 



The ribs are elevated by the action of the external intercostals, the 

 sternal portion of the internal intercostals and the levatores costarum. 

 The external intercostals are situated between the ribs only and are want- 

 ing 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 necessary 

 consequence of their contraction, 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 inspiration, this 

 takes place to such an extent as palpably to carry with it the sternum 

 and the lower ribs. Theoretically, then, the external intercostals can do 

 nothing but render the ribs more nearly horizontal. 



If the external intercostals are exposed in the dog in which the 

 costal type of respiration is quite prominent close observation can 

 hardly fail to show that these muscles enter into action with inspiration. 

 If attention is directed to the sternal portion of the internal intercostals, 



