142 RESPIRATION 



thoracic "pull" exerted upon it, it is normally vaulted upward. 



The antero-posterior diameter is increased chiefly by the ele- 

 vation of the ribs. Since these bones, attached posteriorly to the 

 spinal column, run not only forward but also downward to join 

 the sternum by the costal cartilages, it follows that the elevation 

 of their anterior ends will increase the diameter in question. 



Muscles oj Inspiration. Elevation of the ribs is effected by a 

 number of muscles. The three scaleni are attached above to the 

 cervical vertebrae and below to the first and second ribs; their 

 action elevates not only these ribs but the whole anterior chest 

 wal'. 



The action of the intercostales externi is still a subject of dis- 

 pute in connection with the physiology of respiration. These 

 muscles are attached externally to the adjacent borders of the 

 ribs, and thus occupy the intercostal spaces. Their fibers are 

 directed downward and forward, and the effect of contraction 

 of any single intercostal muscle would be to approximate the 

 two ribs to which it is attached; but if it can be assumed that the 

 first rib is fixed, then, from the direction of their fibers, the ex- 

 ternal intercostals will render the ribs more nearly horizontal by 

 raising their anterior movable extremities. It seems that the 

 first rib is prevented from descending, probably by the simulta- 

 neous contraction of the scaleni. The intercostales interni have a 

 direction almost at right angles to that of the externi; the sternal 

 portions of these act from the sternum and also elevate the 

 anterior extremities of the ribs. The levatores costarum are 

 attached to the transverse processes of the dorsal vertebrae and 

 to the upper borders of the ribs posteriorly. The transverse 

 processes are fixed points and the ribs are movable on their spinal 

 articulations. Contraction of these muscles is, therefore, very 

 efficient in elevating the anterior ends of the ribs. 



The action of the diaphragm is the most notable of the mus- 

 cular phenomena connected with respiration, and it deserves to 

 be called the "muscle of respiration " 



