BY DR. BURDON-SANDERSON. 299 



muscles, this muscular membrane is so thin that they can be 

 easily perceived through it. 



To investigate the part taken by these movements in the 

 mechanism of respiration, it is necessary to ascertain in what 

 relation they stand to the influx and efflux of air. This is 

 accomplished by inserting a suitable glass cannla into one of 

 the nostrils and connecting it with the tympanum, shown in 

 fig. 231. In this way the curve is obtained, which is copied in 

 fig. 240 bis. By watching, at the same time, the motions of 

 the hyoid bone and of the lever, it is easy to satisfy one's self 

 that the retraction of the former towards the sternum corres- 

 ponds with the depression of the latter, and with the entrance 

 of air into the pharyngcal cavity. It is further seen that the 

 motions are by no means uniform, and that in connection with 

 this want of uniformity they present certain peculiarities which, 

 from their intimate connection with the mechanism by which 

 air is introduced into and expelled from the lungs, require 

 careful attention. The tracing enables us to divide the respira- 

 tory acts into two kinds, viz., smaller alternative movements 

 (a a a), which occur at pretty regular short intervals, and larger 

 movements (b b 6), which differ from the others in this respect, 

 that the less energetic expiratory act by which the movement 

 begins, terminates in a sudden expulsion of air, indicated I)}' a 

 more rapid rise of the lever, and determined by a more vigor- 

 ous contraction of those muscles which connect the body of the 

 hyoid bone with the skull. This sudden elevation of the floor 

 of the pharynx is the act by which the frog injects air into its 

 lungs. The student must now fix his attention on the nostrils, 

 when he will see that whereas during the small movements 

 (a a) those organs are motionless, the sudden expulsions (6 b) 

 are accompanied by contraction of the little constrictor mus- 

 cles of the nares, and, consequently, that the latter differ from 

 the former not merely in their greater vigor, but in their being 

 executed with the nostrils more or less closed, so that the air, 

 instead of passing freely out, is injected through the glottis into 

 the lungs. To prove this, watch the expiratory muscles of the 

 flanks (the external oblique particularly). At the first moment, 

 it will perhaps appear as if the sudden contraction of these 

 muscles were coincident with the closure of the nares, but it is 

 soon seen that the former movement follows the latter at an 

 interval of time, which, although very short, is not difficult to 

 appreciate even without instruments. This may be demon- 

 strated graphically by puncturing* the anterior wall of the vis- 

 ceral cavity, and introducing through the puncture a canula 

 in such a way that it communicates with the cavity of one lung. 

 The canula being connected with a tympanum, a tracing is ob- 

 tained, which shows that the period during which the air is 

 contained in the lungs is extremely short, that the entry of air 



