398 COMPARATIVE PHYSIOLOGY. 



More blood, then, entering the right side of the heart dur- 

 ing inspiration, more will be thrown into the systemic circula- 

 tion, unless it be retained in the lungs, and, unless the effect be 

 counteracted, the arterial pressure will rise, and, as all the con- 

 ditions are reversed during expiration, we look for and find 

 exactly opposite results. The lungs themselves, however, must 

 be taken into the account. During inspiration room is pro- 

 vided for an increased quantity of blood, the resistance to its 

 iiow is lessened, hence more blood reaches the left side of the 

 heart. The immediate effect would be, notwithstanding, some 

 diminution in the quantity flowing to the left heart, in conse- 

 quence of the sudden widening of the pulmonary vessels, the 

 reverse of which would follow during expiration ; hence the 

 period of highest intra-thoracic pressure is after the onset of 

 the expiratory act. During inspiration the descent of the 

 diaphragm compressing the abdominal organs is thought to 

 force on blood from the abdominal veins into the thoracic vena 

 cava. 



That the respiratory movements do exert in some way a 

 pronounced effect on the circulation the student may demon- 

 strate to himself in the following ways : 1. After a full inspira- 

 tion, close the glottis and attempt to expire forcibly, keeping 

 the fingers on the radial artery. It may be noticed that the 

 pulse is modified or possibly for a moment disappears. 2. Re- 

 verse the experiment by trying to inspire forcibly with closed 

 glottis after a strong expiration, when the pulse will again be 

 found to vary. In the first instance, the heart is comparatively 

 empty and hampered in its action, intra-thoracic pressure being 

 so great as to prevent the entrance of venous blood by com- 

 pression of the heart and veins, while that already within the 

 organ and returning to it from the liings soon passes on into 

 the general system, hence the pulseless condition. The expla- 

 nation is to be reversed for the second case. The heart's beat is 

 modified, probably reflexly, through the cardio-inhibitory cen- 

 ter, for the changes in the pulse-rate do not occur when the vagi 

 nerves are cut, at least not to nearly the same extent. • 



Comparative. — It may be stated that the cardiac phenomena 

 referred to in this section are much more marked in some ani- 

 mals than in others. Very little change may be observed in 

 the pulse-rate in man, while in the dog it is so decided that one 

 observing it for the first time might suppose that such pro- 

 nounced irregularity of the heart was the result of disease ; 



