396 TEXT-BOOK OF PHYSIOLOGY. 



preponderates in the male, the latter in the female, giving rise to what has been 

 termed in the one case the diaphragmatic or abdominal and in the other the 

 thoracic or costal type of respiration. The cause of this greater mobility 

 and activity of the thorax in the female has been a subject of much discus- 

 sion. It has been attributed, on the one hand, to the necessity for a physi- 

 ologic adjustment between respiration and child-bearing, and therefore a 

 specific sex peculiarity; on the other hand, it has been attributed to persistent 

 constriction of the waist, in consequence of which the full play of the dia- 

 phragm is prevented and the burden of inspiration is thrown on the thoracic 

 muscles. It has been assumed that if inspiration were confined in women 

 to the diaphragm, there would arise in the latter stages of gestation such an 

 increase in intra-abdominal pressure that not only would respiratory ex- 

 changes be interfered with, but fetal life might be unfavorably influenced, 

 if not endangered. Modern investigations have not confirmed this assump- 

 tion, but, on the contrary, have corroborated the view that the preponderance 

 of thoracic movement is due to the influences of dress restrictions, for with 

 their removal the so-called costal type of breathing entirely disappears. 

 While gestation may lead to a greater activity of the thorax, this is but tem- 

 porary, for with its termination there is a return to the diaphragmatic type 

 of breathing. 



Number of Respirations per Minute. The number of respirations 

 which occur in a unit of time varies with a variety of conditions, the most 

 important of which is age. The results of the observations of Quetelet on 

 this point, which are generally accepted, are as follows: 



Age. Respirations per Minute. Age. Respirations per Minute. 



o- i year, 44 20-25 years, 18 .7 



5 years, 26 25-30 years, 15.0 



15-20 years, 20 30-50 years, 17 .o 



From these observations it may be assumed that the average number of 

 respirations in the adult is eighteen per minute, though varying from moment 

 to moment from sixteen to twenty. During sleep, however, the respiratory 

 movements often diminish in number as much as 30 per cent., at the same 

 time diminishing in depth. 



Rhythm. Each respiratory act takes place normally in a regular 

 methodic manner, each event occurring in a definite sequence and occupy- 



FIG. 1 93: PNEUMOGRAPH. (Fitz.) 



ing the same relative period of time. This rhythm, however, is not infre- 

 quently temporarily disturbed by emotions, volitional acts, muscle activity, 

 phonation, changes in the composition of the blood, etc.; with the removal 

 of these disturbing factors, the respiratory mechanism soon returns to its 

 normal condition. 



