THE RESPIRATORY SYSTEM. 403 
oles affected through vaso-motor nerves in obedience to the 
medullary center which operates by their agency; and that 
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Fic. 20,—Tracings of blood-pressure in rabbit to show Traube-Hering curves (after Foster). 
The widest undulations indicate Traube Hering curves; those next in size, effects of 
respiration ; and the smallest, of the pulse. 
when this center is disabled its subordinates in the spinal cord 
take upon them the task. It has also been suggested that there 
may be a local vaso-motor mechanism acted upon by the ve- 
nous blood or that the muscle-cells. themselves may be influ- 
enced by the unnatural condition of the blood in asphyxia. 
These curves, however, also appear during respiration that 
deviates but little from the normal. 
It is to be borne in mind that the tracings on which we 
have based our reasoning do not represent what takes place in 
every mode of breathing. The subject is one of great com- 
plexity. Doubtless mechanical explanations go a long way 
here, but they are so mixed up with factors that play a part 
more or less prominent, though difficult to isolate in individual 
instances, and in no wise to be explained as other than vital 
effects, that one must exercise the usual caution; the more so 
as it is found upon actual experiment that the outcome, as 
regards blood-pressure, is not always quite what would have 
been expected, reasoning from the principles of physics alone, 
That there are rhythms within rhythms in the vascular and 
respiratory system, rendering the subject complex beyond the 
power of experiments fully to unravel, is a conviction that we 
think will deepen in the minds of physiologists. 
