402 ANIMAL PHYSIOLOGY. 
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 comparative- 
ly empty and hampered in its action, intra-thoracic pressure 
being so great as to prevent the entrance of venous blood by 
compression of the heart and veins, while that already within 
the organ and returning to it from the lungs soon passes on 
into the general system, hence the pulseless condition. The 
explanation is to be reversed for the second case. The heart’s 
beat is modified, probably reflexly, through the cardio-inhibitory 
center, 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; 
though even in this animal there are variations in this respect 
with the breed, age, etc. 
We must now direct attention to certain facts which have 
been very differently interpreted. 
During artificial respiration, when air is pumped into the 
chest by a bellows, it follows, of course, that all the usual press- 
ure conditions are reversed—e. g., the inspiratory pressure is 
greater than the expiratory. 
If artificial respiration, in an animal under experiment, be 
stopped, it may be noticed that there is at first a steady rise of 
blood-pressure ; but presently certain undulations in the respir- 
atory tracings may be observed, known as Traube-Hering 
curves; and these will appear even when the vagi nerves are 
cut, and disappear only with the fall of blood-pressure that 
ensues with the exhaustion of the animal. , 
If the spinal cord has been divided, the tracings may still 
be obtained, though the effect is not so marked. These are the 
phenomena, but there is much divergence of opinion as to their 
cause. Some maintain that mechanical effects suffice to explain 
them, though the majority are not of this opinion, but believe 
them due to rhythmical variations in the caliber of the arteri- 
