RESPIRATION 409 



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 17.0 



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- 

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

 infrequently 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 re- 

 turns to its normal condition. 



FIG. 189. PNEUMOGRAPH. (Fitz.) 



A graphic representation of the excursions of the thoracic walls, rhythmic 

 or otherwise, is obtained by fastening to the thorax an apparatus, a stethome- 

 ter or a pneumo graph, which by means of a tambour takes up and trans- 

 mits the movement to a second tambour provided with a recording lever. 

 A simple form of pneumograph, suggested by Fitz (Fig. 189), consists of a 

 coil of wire two and a half centimeters in diameter and about 40 centimeters in 

 length, enclosed by thin rubber tubing, one end of which is closed, the other 

 placed in communication either with a tambour and lever or with a piston 

 recorder. By means of an inelastic cord or chain the apparatus is securely 

 fastened to the chest. With each inspiration the spring is elongated, the air 

 within the system is rarefied, and as a result the lever falls; with each expira- 

 tion the reverse conditions obtain and the lever rises. If the lever be ap- 

 plied to the recording surface of a moving cylinder, a curve of the thoracic 

 movement, a pneumatogram, is obtained (Fig. 190), from which it is appa- 

 rent that inspiration takes place more abruptly and occupies a shorter 

 period of time than expiration; that expiration immediately follows inspira- 

 tion, but that there is a slight pause between the end of the expiration and 



