372 



COMPARATIVE PHYSIOLOGY. 



is free access for the air through the glottic opening, the lungs 

 are of necessity expanded. This ingoing air has had to over- 

 come the elastic resistance of the lungs, which amounts to ahout 



Fig. 2r6.— Apparatus to illnstrate relatione of intra-thoracic and external pressurep. 

 (after Beaunia). A glass bell-jar is provided with a light stopper, through which 

 passes a branching glass tube fitted with a pair of elastic bags representing Inngs. 

 The bottom of the jar is closed by rubber membrane representing diaphragm. A 

 mercury manometer indicates the difference in pressure within and without the 

 bell-jar. In left-hand figure it will be seen that these pressures are equal ; in right 

 (inspiration), the external pressure is considerably greater. At one part (6) an 

 elastic membrane fills a hole in jar, representing an intercostal space. 



five millimetres of mercury in man, as ascertained by tying a 

 manometer in the windpipe of a dead subject, and then opening 

 the thorax to equalize the inside and outside pressures, when 



the lungs at once collapse and 

 the manometer shows a rise of 

 the mercury to the extent indi- 

 cated above. To this we must 

 add the influence of the tonic 

 contraction of the bronchial 

 muscles before referred to, 

 though this is probably not very 

 great. 



That there are variations of 

 intrapulmonary pressure may 

 be ascertained by connecting a 

 manometer with one nostril — 

 the other being closed — or with 

 the windpipe. The mercury 

 shows a negative pressure with 

 each inspiratory, and a positive 



Fig. 296.— Dorsal view of four vertebrae 

 and three attached ribs, showing at- 

 tachment of elevator muscles of ribs 

 and intercostals (after Allen Thom- 

 son). 1, long and short elevators; 2, 

 external intercostal; 3, internal in- 

 tercostal. 



