870 ANIMAL PHYSIOLOGY. 
robbed of its oxygen, and rendered impure by the addition of 
carbonic dioxide, the former must be renewed and the latter 
expelled; and,;as mere diffu- 
sion takes place too slowly to 
accomplish this in the mam- 
mal, this process is assisted 
by the nervous system set- 
ting certain muscles at work 
to alter the size of the chest 
cavity. Because of the ribs 
being placed obliquely, it fol- 
lows that their elevation will 
result in the enlargement of 
the thoracic cavity in the an- 
tero-posterior diameter; and, 
as the chest, in consequence, 
gets wider from above down- 
ward, also in the transverse 
diameter; which is more- 
over assisted by the eversion 
of the lower borders of the 
Fro, 20 Diagrarn, ustating clevatien of rts ribs; and, if the convexity of 
papa fhonts, sternum, and costal cartilages the diaphragm were dimin- 
ished by its contraction and 
consequent descent, it would follow that the chest would be in- 
creased in the vertical diameter also. All these events, favor- 
able to the entrance of air, actually take place through agencies 
we must now consider. The student is recommended to look 
into the insertion, etc., of the muscles concerned, to which we 
can only briefly refer. 
The act of inspiration commences by the fixation of the 
uppermost ribs, beginning with the first two, by means of the 
scaleni muscles, this act being followed up by the contraction 
of the external intercostals, leading to the elevation of the 
other ribs; at the same time, the arch of the diaphragm de- 
scends in consequence of the contraction of its various mus- 
cular bundles. Under these circumstances, the air from with- 
out must rush in, or a vacuum be formed in the thoracic 
cavity; and, since there is free access for the air through the 
glottic opening, the lungs are of necessity expanded. This in- 
going air has had to overcome the elastic resistance of the 
lungs, which amounts to about 5 millimetres of mercury in 
man, as ascertained by tying a manometer in the windpipe of 
