3S4 TEXT-BOOK OF PHYSIOLOGY. 



the membranes are widely separated and almost stationary, seldom 

 moving in either direction more than a few millimeters. In labored 

 respirations these movements are naturally increased in extent. The 

 reflex movements of the membranes occasioned by the unskilful use 

 of the laryngoscope, especially with nervous patients, are not to be re- 

 garded as strictly physiologic. The respiratory space in quiet breath- 

 ing is an isoceles triangle, with a length of 20 mm. and a width at the 

 base of 15.5 mm. 



Respiratory Types. Observation of the respiratory movements 

 in the two sexes shows that while the enlargement of the thoracic 

 cavity is accomplished both by the descent of the diaphragm (as 

 shown by the protrusion of the abdomen) and the elevation of the 

 thoracic walls, the former movement 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 

 physiologic 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 diaphragm 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 exchanges 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 pre- 

 ponderance of thoracic movement is due to the influences of dress 

 restrictions, for with their removal the so-called costal type of breath- 

 ing entirely disappears. While gestation may lead to a greater 

 activity of the thorax, this is but temporary, for with its termination 

 there is a return to the diaphragmatic type of breathing. 



Number of Respirations per Minute. The number of respira- 

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

 the most important of which is age. The results of the observa- 

 tions of Quetelet on this point, which are generally accepted, are as 

 follows : 



A se. Respirations per Minute. Age. Respirations per Minute. 



o- i year, 44 20-25 years, 18.7 



5 years, _ ___ 2 6 25-30 " 16.0 



r 5-20 _ 20 30-50 " 18.0 



From these observations it may be assumed that the average number 

 of respirations in the adult is eighteen per minute, though varying 



