400 ANIMAL PHYSIOLOGY. 
Urine is secreted in excess, there is more muscular energy, and 
the metabolism of the body generally is accelerated. Air under 
altered pressure has been employed as a therapeutic agent, but 
a little reflection will make it clear that it is a remedy to be 
used with the greatest care, especially when there is disease of 
the heart, blood-vessels, etc. 
THE INFLUENCE OF RESPIRATION ON THE CIRCULATION. 
An examination of tracings of the intra-thoracic and blood- 
pressure, taken simultaneously, shows (1) that during inspira- 
tion the blood-pressure rises and the intra-thoracic pressure 
falls; (2) that during expiration the reverse is true; and (2) that 
the heart-beat is slowed, and has a decided effect on the form 
of the pulse. But it also appears that the period of highest 
blood-pressure is just after expiration has begun. 
i e a e 
Fie. 319.- Tracings of blood-pressure and intrathoracic pressure (after Foster). a, blood- 
pressure tracing showing irregularities due to respiration and pulse; 0b, curve of intra- 
thoracic pressure ; 7, beginning of inspiration ; e, of expiration. Intrathoracic pressure 
is seen to rise rapidly after inspiration ceases, and then slowly sinks as the expiratory 
blast continues, to become a rapid fall when inspiration begins. 
We must now attempt to explain how these changes are 
brought about. By intra-thoracic pressure is meant the press- 
ure the lungs exert on the costal pleura or any organ within 
the chest, which must differ from intra-pulmonary pressure 
and the pressure of the atmosphere, because of the resistance 
of the lungs by virtue of their own elasticity. 
It has been noted that even in death the lungs remain par- 
tially distended; and that when the thorax is opened the pul- 
monary collapse which follows demonstrates that their elas- 
ticity amounts to about five millimetres of mercury, which 
must, of course, represent but a small portion of that elasticity 
which may be brought into play when these organs are greatly 
distended, so that they never press on the costal walls, heart, 
