The influence of these two factors - the heart and the vessels - on the blood-pressure varies in different animals, and under different conditions; and a number of the discrepancies observed by various investigators are probably due to this circumstance. Thus, in dogs the effect of the heart is very considerable, and when its beats are quickened by division of the vagi the pressure rises; in rabbits, on the other hand, the heart, instead of working well under its power as in the dog, beats very rapidly in the normal condition, and when the vagi are divided the pressure does not rise much, although when they are stimulated the pressure falls both in the dog and in the rabbit. This different action of the vagus in the dog and rabbit is well seen when these animals are poisoned by atropine. This drug completely destroys the inhibitory action of the vagus on the heart; and when the inhibitory power is completely removed we find that only a slight increase in the number of beats takes place in the rabbit, the pulse-rate rising one quarter: for example, perhaps from 100 to 125. In the dog, on the contrary, the pulse-rate will rise to three times, or even four times, what it was before.

1 Dogiel, Pfluger's Archiv, 1874, Bd. viii.

In man the effect of the vagus on the heart is intermediate between that of the rabbit and dog: so that if the normal pulse is between 70 and 80 in the minute, it rises to between 140 and 180 when the vagus is paralysed by atropine (Von Bezold).

This difference between the effect of the vagus on the heart alters the effect of drugs on the blood-pressure in different animals.

The difference in the action of drugs on the dog and rabbit is well shown in the case of nitrite of amyl. If this be given by inhalation to a rabbit, the blood-pressure falls immediately and rapidly. If given to a dog the fall may be very slight, at least if a small quantity only is used. On counting the pulse in the dog we discover at once the cause of the apparent difference in the action of the drug on the two animals. Before inhalation the pulse of the dog was slow, but after inhalation its pulse became almost as quick as that of the rabbit. In both animals the nitrite causes dilatation of the vessels, but in the dog the heart begins to beat so much more rapidly than usual that it maintains the blood-pressure nearly at the normal, notwithstanding this dilatation; while the heart of the rabbit beats so quickly, normally, that it cannot maintain the pressure by increased rate of pulsation. If the vagi be cut in the dog, so that the heart beats rapidly like that of the rabbit before inhalation, the nitrite causes as sudden a fall as in the rabbit.1

The numerous factors which have to be taken into consideration in regard to the blood-pressure, the action and the interaction of different parts of the body upon one another, render it by no means easy to understand the effect of drugs on the circulation. The differences which we find in the action of drugs on different animals seem at first to make matters still worse; but it is through these differences of action that we learn the exact mode in which the various factors of the circulation are affected by the drug.

There are at least two other factors which must be borne in mind in relation to the difference between rabbits and dogs: these are (1) the much greater sensitiveness of the inhibitory nerves of the heart to reflex stimulation from the nose as well as to stimulation by venous blood, in the rabbit than in the dog; and (2) the proportionately much greater length of the intestinal tube in the rabbit, which causes the vessels of the intestines, on account of their number, to exercise a greater action on the blood-pressure in it than in the dog. Thus, in the rabbit, a slightly irritating vapour will cause the animal to close it3 nostrils; and almost immediately the vagus will be excited and the heart will stop. This stoppage is probably chiefly due to reflex action on the heart through the nasal nerves, though it may be partly due to accumulation of carbonic acid in the blood. When the spinal cord is divided in the rabbit just below the medulla, the pressure sinks enormously: in the dog it also sinks, but not to the same extent; and in some cases it sinks so little that it is almost impossible to believe that the cord has been divided, until examination after death shows that the section has really been completed. This effect may be partially due to the less power which the dilatation of the intestinal vessels, consequent upon the section, has in the dog. It may also, however, be partly due to greater development of extra-cranial vaso-motor centres in the spinal cord and elsewhere, than in the rabbit.

1 Lauder Brunton, Journ. of Anat. and Physiol., Nov. 1870, p. 95.