THE RESPIRATORY SYSTEM. 



397 



which may be brought into play when these organs are greatly 

 distended, so that they never press on the costal walls, heart, 



Fig. 310. — Tracings of blood-pressure and intrathoracic pressure in the dog (after 

 Foster), a, blood-pressure tracing showing irregularities due to respiration and 

 pulse; b, curve of intrathoracic pressure; i, beginning of inspiration; e, of expira- 

 tion. Intrathoracic pressure is seen to rise raj)idly after inspiration ceases, and 

 then slowly sinks as the expiratory blast continues, to become a rapid fall when 

 inspiration begins. 



etc., with a pressure equal to that of the atmosphere. It follows 

 that the deeper the inspiration the greater the difference be- 

 tween the intra-thoracic and the atmospheric pressure. Even 

 in expiration, except when forced, the intra-thoracic pressure 

 remains less, for the same reason. 



These conditions must have an influence on the heart and 

 blood-vessels. Bearing in mind that the pressure without is 

 practically constant and always greater than that within the 

 thorax, the conditions are favorable to the flow of blood toward 

 the heart. As in inspiration, the pressure on the great veins 

 and the heart is diminished, and, as these organs are not rigid, 

 they tend to expand within the thorax, thus favoring an on- 

 ward flow. But the opposite effect would follow as regards the 

 large arteries. Their expansion must tend to withdraw blood. 

 During expiration the conditions are reversed. The effects on 

 the great veins can be observed by laying them bare in the 

 neck of an animal, when it may be seen that during inspiration 

 they become partially collapsed, and refilled during expiration. 

 In consequence of the marked thickness of the coats of the 

 great arteries, the effect of changes in intra-thoracic pressure 

 must be slight. The comparatively thin-walled auricles act 

 somewhat as the veins, and it is likely that the increase of 

 pressure during expiration must favor, so far as it goes, the car- 

 diac systole. 



