EXTERNAL RESPIRATION AND RESPIRATORY MOVEMENTS. 573 



Fig. 235.- 



Sixth dorsal vertebra and 

 rib. (Reichert.) 



brought into play to prevent this result. As stated below, the 

 quadratus lumborum and the serratus posticus inferior may have 

 this function of fixating the lower 

 ribs in violent inspirations. The 

 diaphragmatic muscle is innervated 

 on each side by the corresponding 

 phrenic nerve. This nerve arises 

 in the neck from the fourth and 

 fifth cervical spinal nerves, and 

 passes downward in the chest in 

 the mediastinal space, lying close 

 to the heart in part of its course. 

 Section of this nerve paralyzes, of 

 course, the diaphragm on the cor- 

 responding side. 



Elevation of the Ribs. As a 

 necessary result of the structure of 

 the bony thorax, every elevation 

 of the ribs must cause an enlarge- 

 ment of the thoracic cavity in the 

 dorsoventral and the lateral diam- 

 eters. We are justified in saying 

 that every muscle whose contrac- 

 tion causes an elevation of the ribs is an inspiratory muscle. This 

 result is due, in the first place, to the slant 

 of the ribs. Each rib is attached to the 

 spinal column at two points : the head to 

 the body of the vertebra and the tubercle 

 to the transverse process. The up-and- 

 down movements of the ribs may be re- 

 garded as rotations around an axis joining 

 these two points, that is, each point in 

 the rib as it moves up or down describes 

 a circle around this axis (see Fig. 235). 

 If our ribs were set upon the vertebral col- 

 umn so that the plane of the rib formed a 

 right angle with the column, then every 

 movement of the rib up or down would 

 decrease the size of the thorax and there- 

 fore cause an expiration. As a matter 

 of fact, however, the ribs slant downward, 

 so that if elevated the sternal end is car- 

 ried farther away from the sternum and 

 the chest is enlarged in the dorsoventral 

 Moreover, as the rib moves upward there 



Fig. 236. Diagram to il- 

 lustrate the effect of the slant 

 of the ribs: S, The spinal col- 

 umn; a, the position of the 

 rib in normal expiration; (a') 

 its position (exaggerated) in 

 inspiration (the distance be- 

 tween the spinal column and 

 the sternum (st.), the antero- 

 posterior or dorsoventral di- 

 ameter of the chest is in- 

 creased) . Any movement from 

 the position a' would cause an 

 expiration. 



direction (see Fig. 236). 



