THE MECHANICS OF THE RESPIRATORY MOVEMENTS 



467 



Under ordinary conditions, however, these terms are used only in a 

 relative way to indicate whether the diaphragm and lower intercostals 

 or the upper costal and intercostal muscles are the chief factors in 

 respiration. It is customary to state that the respiration in men is 

 diaphragmatic, while that of women is costal. ^ By some investigators 

 this greater mobility of the thorax in the female is regarded as a sexual 

 characteristic, while others hold that it is the artificial result of a 

 long-continued constriction of the waist and encasement of the abdo- 

 men. If regarded as a sexual characteristic, it may be said to pro- 

 tect the developing fetus against an undue intra-abdomi- 

 nal pressure, and to help the mother in retaining her 

 normal respiratory capacity. The more recent investi- 

 gations, however, do not uphold this view, because it 

 has been shown that Indian and Chinese women, who 

 have never worn corsets, show the abdominal type of 

 breathing. 2 This is also true of white women who have 

 not been in the habit of wearing tight-fitting clothes or 

 have discarded the corset in later years. Thus, while it 

 seems certain that the preponderance of costal breath- 

 ing in woman does not possess a physiological basis, 

 gestation no doubt throws the burden of respiration 

 temporarily upon the thorax, until at the end of this 

 period the play of the diaphragm can again go on un- 

 hinderedly. In visceroptosis the action of the diaphragm 

 is greatly restricted and costal breathing is brought 

 into play more and more. 



In some animals the diaphragm presents a rounded 

 outline and its tendinous portion is placed in the center 

 between the sternum and the vertebral column. In 

 others it is arranged bilaterally, its antero-posterior di- 

 ameter in the region of the sternum being shorter than 

 its transverse or oblique diameter. Each half is inner- 

 vated by the corresponding phrenic nerve which arises 

 from the third and fourth cervical spinal nerves and 

 passes downward through the thorax in close proximity 

 to the heart. The division of one of these nerves is 

 immediately followed by a paralysis of the corresponding half of the 

 diaphragm, while the division of both nerves gives rise to a uselessness 

 of the entire muscle. The latter procedure generally proves fatal, and 

 especially in young animals, because the respiratory interchange then 

 becomes wholly inadequate. In this connection mention might also 

 be made of the fact that the quiescent diaphragm contracts at times 

 synchronously with the heart. This phenomenon, which may be ob- 

 tained with the chest open or closed, seems to require a certain high 

 degree of nervous excitability, either local or general. The action 



1 Hasse, Archiv fiir Anatomie, 1903, 23. 



2 Fitz, Jour, of Exp. Med., i, 1896. 



Fig. 242. 



D lAGBAM 



Showing the 

 Enlargement 

 OF THE Trunk 



ON NoRMAIi 



(Diaphragma- 

 tic) AND For- 

 ced (Costal) 

 Respiration. 



