418 



PHYSIOLOGY 



CHAP. 



average increases only some 2*5 mm. on every side by the action 

 of the external intercostals, this still, according to Fick, yields a 

 volume increase of 500 c.c., which corresponds to the average 

 volume of normally inspired air. According, therefore, to Fick's 

 calculations, the diaphragm hardly takes any part in normal 

 inspiration, or at most the tonicity of its fibres is augmented, 

 in order to check any upward aspiration during the widening of 

 the thorax. 



This theory is evidently exaggerated, and does not harmonise 

 with the fact that in the quiet breathing of man the maximal 

 excursion of the sagittal diameter is observed to correspond with 

 the epigastrium (Fig. 183), which can only result from the 

 inspiratory activity of the diaphragm. The fact, however, remains 



A' 



FK;. 184. Thoracic and abdominal pneumograms during waking (T and A) and sleep (T' A'). 

 (A. Mosso.) The curves are reversed, T and T being traced with Marey's pnenmograph, A A' 

 with Vierordt's sphygmographic lever applied near the umbilicus. 



that the function of the diaphragm is normally far less im- 

 portant than that of the external intercostals taken as a whole. 

 Hultkrantz has recently shown that in an individual who 

 takes in on an average 490 c.c, of air, 320 c.c. are to be referred 

 to thoracic dilatation, and only 170 c.c. to the depression of the 

 diaphragm. 



Again, it appears from certain curves of thoracic and abdominal 

 respiration which Mosso recorded simultaneously in the waking 

 and the sleeping states, that the characteristic abdominal type of 

 human respiration in the waking state disappears in sleep, during 

 which the activity of the intercostals increases, while that of the 

 diaphragm is reduced to a minimum (Fig. 184). 



As regards the question whether during normal quiet breathing 

 the external intercostals only come into play by the raising of the 

 ribs, or whether the scaleni and levatores costarum breves et longi 



