574 . PHYSIOLOGY OF RESPIRATION. 



- f m ^s I 



is^ifi obvious enlargement of the chest in the lateral diameter. 



result may be referred to two causes : In the first place, the 



of the rotation of the ribs, that is, the line joining the head 



id the tubercle of the rib is inclined downward so that the plane 



rotation, which is, of course, at right angles to this axis, will be 

 inclined outward. As the rib is moved upward, therefore, it must 

 also move outward. Secondly the cartilaginous ends of the ribs are 

 fixed at the sternum so that as they move upward and outward 

 they will be twisted or everted somewhat in the middle, with a 

 torsion of the cartilaginous ends. 



The Muscles of Inspiration. In addition to the diaphragm, 

 all muscles attached to the thorax whose contraction causes an 

 elevation of the ribs must be classed as inspiratory muscles. In 

 regard to this latter group the action of some of them is either 

 evident from their anatomical attachments, or the muscles may be 

 stimulated directly and the effect of their contraction noted. In 

 other cases, however, it is necessary to make use of the method 

 first suggested by Newell Martin, namely, to determine whether 

 the contraction of the muscle in respiration occurs simultaneously 

 with that of the diaphragm or alternately with it. In the former 

 case it is inspiratory, in the latter expiratory. The following mus- 

 cles may be classed as inspiratory : Levatores costarum. They arise 

 from the transverse process of the seventh cervical and first to 

 eleventh thoracic vertebrae and are inserted into the next- rib or the 

 second rib below. Intercostales externi muscles. They lie in the inter- 

 costal spaces extending from the lower edge of one rib to the upper 

 edge of the rib below ; they slant downward and toward the mid-line. 

 These muscles have been assigned different functions by different 

 authors, but the experiments made by Hough,* using the method 

 of Martin described above, show that they are inspiratory. It 

 was found that in the dog they contract synchronously with the 

 diaphragm. The same authors find that the intercartilaginous 

 portions of the internal intercostals are also inspiratory. The 

 scaleni anterior, medius, and posterior arise from the transverse 

 processes of the cervical vertebrae and are inserted into the first and 

 second ribs. M. sterno-cleido-mastoideus extends from the mastoid 

 process to the sternum and sternal extremity of the clavicle. M. 

 pectoralis minor extends from the coracoid process of the scapula 

 to the anterior surface of the second to the fifth rib. M. serratus 

 posticus superior extends from the spinous processes of the lower 

 cervical and upper dorsal vertebrae to the second to fifth rib. 



The Muscles of Expiration. Expiration that is, diminution 



* Hough, "Studies from the Biological Laboratory, John Hopkins 

 University," 5, 91, 1893, and Bergendal and Bergman, " Skandinavisches 

 Archiv f. Physiologie, " 7, 178, 1896. 



