308 RESPIRATION. 



of the aryteenoid cartilages are well seen the chink becoming 

 wider in inspiration, narrower in expiration. To observe the 

 motions of the arytrenoid cartilages, the best way is to excite 

 the recurrent nerves, when it is seen that during excitation 

 the vocal cord of the same side approaches the middle line. 

 If both recurrents are excited, the riraa is completely closed, 

 the arytrenoid cartilages applying themselves to each other 

 just as they do in the production of a musical note. Con- 

 sidering that the recurrent nerve is distributed to all the 

 muscles, and not merely to those which act as constrictors 

 (arytsenoidei and crico-arytsenoidei laterales), and that the 

 movements produced are of the same nature as those which 

 occur in ordinary expiration, though much more vigorous, we 

 arrive at the inference that in both cases the widening of the 

 glottis is a condition of general muscular relaxation, or, in 

 other words, that all the intrinsic muscles of the larynx are 

 expiratory their combined effect manifesting itself in ap- 

 proximation of the vocal cords, not because the posterior 

 crico-arytii'noid muscles and the other dilating muscles do not 

 act with the rest, but because they are overpowered by them. 



SECTION III. MEASUREMENT OF THE QUANTITY OF AIR RESPIRED IN 



A GIVEN TIME, AND OF THE VOLUME OF AlR INHALED IN EACH RE- 

 SPIRATORY ACT. 



95. The apparatus for this purpose consists of three parts, 

 viz., (a) a receiver or chamber in which the air to be breathed 

 during the period of observation is contained; (b) a face-piece 

 and tube for connecting the receiver with the respiratory 

 cavity of the subject of observation ; (c) arrangements for 

 supplying fresh air to the receiver, to take the place of the 

 air breathed. 



To obtain results which are reliable, the first and most im- 

 portant condition is that the air should be respired without 

 the slightest effort. To insure this, the receiver must be of 

 such construction that the pressure to which the air contained 

 in it is subjected should be the same as that of the atmos- 

 phere. Consequently, it must have the form either of a gaso- 

 meter, the cylinder of which is accurately counterpoised, or 

 that of a membranous bag, the material of which is so thin 

 that H offers no resistance either in contracting or expanding. 

 The l>est material for the latter purpose is vulcanized India- 

 rubber; it is, however, difficult to obtain bags of this descrip- 

 tion which are perfect. Whatever be its form, the receiver 

 must have two openings, one communicating with the face- 

 piece, the other for the reception of air. It must be also so 

 constructed that the moment at which it is full may be easily 

 and accurately observed. 



