142 RESPIRATION 



thoracic diameters expand the thorax. The diameters in- 

 creased are chiefly the (i) vertical, and (2) ant ero- posterior. 



The vertical is increased by descent of the diaphragm, 

 which descent is caused by its contraction, since, owing to 

 the intra-thoracic "pull" exerted upon it, it is normally 

 vaulted upward. 



The antero-posterior diameter is increased chiefly by the 

 elevation of the ribs. Since these bones, attached posteriorly 

 to the spinal column, run not only forward but also down- 

 ward to join the sternum by the costal cartilages, it follows 

 that the elevation of their anterior ends will increase the di- 

 ameter in question. 



Muscles of 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 sec- 

 ond ribs; their action elevates not only these ribs but the 

 whole anterior chest wall. 



The action of the intercostales externi is still a subject of 

 dispute in connection with the physiology of respiration. 

 These muscles are attached externally to the adjacent bor- 

 ders 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 external intercostals will 

 render the ribs more nearly horizontal by raising their an- 

 terior movable extremities. It seems that the first rib is pre- 

 vented from descending, probably by the simultaneous con- 

 traction of the scaleni. The intercostales interni have a di- 

 rection almost at right angles to that of the externi ; the ster- 

 nal 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 verte- 

 brae and to the upper borders of the ribs posteriorly. The 

 transverse processes are fixed points and the ribs are mov- 



