316 RESPIRATION 



they can, with very slight contraction, produce very marked movements of the 

 anterior ends. 



In the rabbit with more vigorous respiration the serrati postici superiores, 

 the sternohyoidei and the sternothyroidei come into play. In man, finally, 

 Duchenne has found that in the greatest respiratory distress the following mus- 

 cles are active : the sternocleido mastoids which lift the sternum when the head is 

 fixed; the pectorales minores which lift the third to the fifth ribs with the scapula 

 fixed; the serrati antici magni, the pectorales majores and the subclavii. 



C. MOVEMENTS OF THE DIAPHRAGM 



The diaphragm springs from the entire inner surface of the lower edge of 

 the thoracic skeleton; its fibers converge toward the axis of the body, and attach 

 themselves to the flat tendon situated in the center of the muscle. It presents 



FIG. 125. Schema, after Hasse, showing the movements of the diaphragm, liver, stomach, 



and spleen in respiration. 



a convex curvature toward the thoracic cavity, being, so to speak, arched over 

 the convex upper surface of the liver. 



When the muscle fibers of the diaphragm contract, its dome-shaped upper 

 part is flattened and moves downward. The central tendon takes part in 

 the movement and becomes flattened because of the pull of the muscle fibers 

 on all sides of its periphery. However, in deep respiration the dome itself 

 always descends further than does the center (Hasse, Fig. 125). According 

 to observations made with X rays (Cowl), during deep respiration the sweep 

 of the diaphragm corresponds to the distance from the middle of the tenth 

 to the upper edge of the twelfth thoracic vertebra. The maximal excursion 

 of the central tendon is about 4 cm. (Gronroos). 



At the same time by elevation of the ribs and of the sternum, the lower 

 end of the thorax is increased in diameter (Duchenne). This is possible 

 because the abdominal viscera, although depressed as an entire mass by the 

 contraction of the diaphragm, present their upper surface as a fulcrum on 

 which the circumference of the diaphragm is lifted. If the abdominal viscera 

 be removed, when the diaphragm contracts the lower ribs approach each other 

 and the lower end of the thorax is narrowed. 



By reason of these changes the thoracic activity is enlarged from above 

 downward and, at the extreme lower end, is enlarged also from side to side. 



