THE RESPIRATORY SYSTEM. 871 
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 ex- 
tent indicated above. To this 
we must add the influence of 
the tonic contraction of the 
bronchial muscles before re- 
ferred to, though this is prob- 
ably not very great. 
That there are variations 
of intrapulmonary pressure 
may be ascertained by con- 
necting a manometer with one 
nostril—the other being closed 
—or with the windpipe. The 
mercury shows a negative 
pressure with each inspirato- 
ry, and a positive with each 
expiratory act. This may 
amount to from 30 to 70 mil- 
limetres with strong inspira- 
tion, and 60 to 100 in forcible 
expiration. 
Fic. 301. — Diagrammatic representation of 
action of diaphragm. in inspiration (Her- 
mann). Vertical section through second 
rib on right side. The broken and dotted 
lines show the amount of the descent of the 
diaphragm in ordinary and in deep inspira- 
ion. 
When inspiration ceases, the elastic recoil of the rib carti- 
lages and the ribs themselves, and of the sternum, the weight 
Fie. 302.—Apparatus to illustrate relations of intra-thoracic and external pressures (after 
Beaunis). A glass bell-jar is provided with a light stopper, through which passes a branch- 
ing glass tube fitted with a pair of elastic bags representing lungs. 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 consid- 
erably greater. At one part (6) an elastic membrane fills a hole in jar, representing an 
intercostal space. 
