558 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



thin-walled and slightly distended pulmonary veins more than the thick-walled 

 and more distended pulmonary arteries, so that the "driving force " of the lung 

 circulation, which is essentially the difference in pressure between the blood in 

 the pulmonary arteries and that in the veins, is thereby increased during inspi- 

 ration and the blood-current is driven with greater velocity. More blood thus 

 being brought into the chest, and consequently to the heart, during inspiration, 

 and less resistance being offered to the flow of the blood through the lungs, 

 more blood must ultimately find its way to the left side of the heart, and con- 

 sequently into the general circulation. If, therefore, the general capillary 

 resistance in the systemic circulation remains the same, it is evident that an 

 increased blood-supply to the left ventricle must cause the general blood-pres- 

 sure to rise. That this rise does not become manifest immediately at the 

 beginning of inspiration is doubtless owing to the filling of the flaccid and 

 partially collapsed large veins and to the dilatation of the pulmonary capil- 

 laries. The continuance of the rise for a short time after the cessation of in- 

 spiration is due apparently to the partial emptying of the now distended lung- 

 vessels, whereby, owing to the arrangement of the heart-valves, the excess of 

 blood is forced toward the left side of the heart. 



Besides the above factors, the flow of blood to the right side of the heart is 

 favored by the pressure transmitted from the conjoint actions of the diaphragm 

 and the abdominal walls through the abdominal viscera to the abdominal 

 vessels. The pressure upon the arteries tends to drive the blood toward the 

 lower extremities and to hinder the flow from the heart ; in the veins, however, 

 the flow toward the heart is encouraged, while that from the extremities is 

 hindered. The rigid walls of the arteries protect them from being materially 

 affected, but the flaccid veins are influenced to a marked degree ; while, there- 

 fore, the flow from the left side of the heart is not materially interfered with, 

 that through the veins toward the right side is appreciably facilitated, and thus 

 the supply of blood to the heart is increased. The effects of these movements 

 may be seen after section of the phrenic nerves, which causes paralysis of the 

 diaphragm, when it will be noted that the blood-pressure curves are much re- 

 duced. This diminution is attributed to two causes the enfeebled respiratory 

 movements, which are now confined to the ribs and the sternum, and the 

 absence of the pressure transmitted from the diaphragm through the abdominal 

 organs to the veins. If in such an animal the abdomen be periodically com- 

 pressed, in imitation of the effects produced by the contraction of the dia- 

 phragm, the respiratory curves may be restored to their normal height. 



During expiration, since the conditions are reversed the effects also must be 

 reversed. The increased negative intrathoracic pressure occasioned by inspira- 

 tion now gives place to a gradual diminution, and with this a lessening of the 

 aspiratory action due to the sub-atmospheric intrathoracic pressure ; the blood- 

 supply is further reduced because of the lessened amount of blood coming 

 through the inferior vena cava; the abdominal veins, instead of being com- 

 pressed and their contents forced chiefly toward the heart, are now being 

 filled ; finally, during the shrinkage of the lungs the intrapulmonary vessels 



