232 ANIMAL PHYSIOLOGY. 
‘is actually found, that the pressure varies for different animals, 
size having, of course, in most instances a relation to the result. 
It has been estimated that in the carotid of the horse the arte- 
rial pressure is 150 to 200 mm. of mercury, of the dog 100 to 175, 
of the rabbit 50 to 90. Man’s blood-pressure is not known, but 
is probably high, we may suppose not less than 150 to 200 mm. 
After the fact that there is a certain considerable blood- 
pressure, the other most important one to notice is that this 
blood-pressure is constantly varying during the experiment, 
and, as we shall give reason to believe, in the normal animal ; 
and to these variations and their causes we shall presently turn 
our attention. 
THE HEART. 
The heart, being one of the great centers of life, to speak 
figuratively, it demands an unusually close study. 
THE Carpiac MOVEMENTS. 
There is no special difficulty in ascertaining the outlines of 
the heart by means of percussion on either the dead or the 
living subject. Quite otherwise is it with the changes in form 
which accompany cardiac action. Attempts have been made to 
ascertain the alterations in position of the heart with respect 
to other parts, and especially its own alterations in shape dur- 
ing a systole, the chest being unopened, by the use of needles 
thrust into its substance through the thoracic walls; but the 
results have proved fallacious. Again, casts have been made 
of the heart after death, in a condition of moderate extension, 
prior to rigor mortis ; and also when contracted by a hardening 
fluid. These methods, like all others as yet employed, are open 
to serious objections. 
Following the rapidly beating heart of the mammal with 
the eye produces uncertainty and confusion of mind. We look 
to instantaneous photography to furnish a possible way out of 
the difficulty. 
It may be very confidently said that the mode of contrac- 
tion of the hearts of different groups of vertebrates is variable, 
though it seems highly probable that the divergences for mam- 
mals are slight. The most that can be certainly affirmed of 
the mammalian heart is, that during contraction of the ventri- 
cles they become more conical; that the long diameter is not 
appreciably altered; that the antero - posterior diameter is 
