70 
TRANSACTIONS OF THE TEXAS ACADEMY OF SCIENCE. 
ring at the same time, even though the microscope be used (and by the 
aid of a micrometer) to watch alterations in the size of said arterioles 
under the influence of drugs. The results alleged to be obtained in these 
cases may be attributed largely to the imagination of the observer, and 
the method, therefore, is unreliable. Two, then, are the principal factors 
to be considered in explaining changes of blood-pressure; one relating to 
an action on the heart itself or its nerve mechanism, and the other relat¬ 
ing to an influence exercised on the vaso-motor system which directly or 
indirectly may alter the caliber of the blood-vessels. But not only must 
the changes of the pressure be noticed, the rate of the pulse should sim¬ 
ilarly be observed at the same time for various reasons. 
When in the intact animal the pneumogastric nerves are stimulated by 
an electrical current, the heart-beats are diminished in number and the 
blood-pressure falls. The contrary takes place when the vagi are simply 
divided; under these circumstances the pulse-rate is increased and the 
arterial pressure rises. It is assumed that here it is the heart or its con¬ 
trolling nerve-mechanism that is being directly influenced. A graphic 
Fig. P. 
Tracing III.—The same, 7 minutes later, or 15 minutes after the injection. 
record taken in either of these instances would show a more or less par¬ 
allel course of two curves, one representing the height of the pressure 
and the other the rapidity of the heart’s action. 
On the other hand, if a sensitive nerve be stimulated, by which the 
medullary vasometer centers are roused into activity, and hence contrac¬ 
tion of the arterioles is obtained, by and by an elevation of the pressure 
is noticed, but pari passu with this rise there occurs a diminution in the 
pulse-rate; that is, the increased blood pressure per se is sufficient to ex¬ 
cite the medullary cardio-inhibitory centers, and hence slowing of the 
cardiac beats is obtained. Vice versa , if the arterioles be dilated, as a 
consequence of a vaso-motor paralysis, the blood pressure falls, and un¬ 
der these circumstances the vagi centers, then deprived of a proper 
amount of blood, the natural stimulant, are freed from inhibition, and 
