664 
PROFESSOR H. N. MARTIN ON THE INFLUENCE OF VARIATIONS 
much reserve from the heart of the Frog to that of the Mammal when the direct effect 
of temperature variations is concerned. The Frog can hardly be said to have any 
normal temperature, and has but slightly developed temperature-regulating physio¬ 
logical mechanisms; its healthy temperature varies from a very low point in 
midwinter to 32° C. or above on a Baltimore summer day. The Mammal, on the 
contrary, is constructed to maintain a definite normal temperature, which does not 
vary beyond very narrow limits ; a departure of even a couple of degrees from tins 
normal is always the sign or the cause of pathological processes. We find the 
Mammalia, accordingly, provided with highly complex temperature - regulating 
mechanisms, in possessing winch they differ very sharply from the Amphibia. 
While it might therefore be expected d priori that the Frog’s heart is so constructed 
as to work better at those warm temperatures at which the general nervo-muscular 
apparatus of the animal is most active, and the calls upon the organs of nutrition 
greatest, the discovery that such is actually the case and that the warmed hearts of 
Frogs beat quicker than cold, does not justify us in forthwith concluding that the 
Mammalian heart, placed in and adapted to the needs of an animal with only one 
healthy temperature, would behave in like manner. This doubt concerning the 
validity when extended to warm-blooded animals of arguments based on experiments 
made with the hearts of Frogs is increased when we call to mind the fact that an 
elevation, within physiological limits, of the temperature of the medium to which a 
cold-blooded animal is exposed increases its tissue metamorphoses, as evidenced by 
a greater excretion of carbon oxide, while exactly the reverse is the case in respect 
to the Mammal. Recalling the wonderful physiological adaptation of the organs of 
animals to the conditions under which they live, we might almost expect that 
increased temperature (not reaching pathological limits) of the blood earned to it 
would lead to a slowing 1 of the beat of the Mammalian heart in correlation with 
the diminished oxidations then occurring in the body generally, and its consequently 
diminished nutritional demands. 
There are still other reasons why the direct application to the Mammalian heart of 
the results of experiments upon Frogs is unsatisfactory. It has been shown (4) that 
the muscular tissue of the Amphibian heart differs considerably in histological 
characters from that of the Mammalian : with this difference in minute structure quite 
important functional differences may be associated. Moreover, the Mammalian heart 
is known to be far more under the control of extrinsic nerve centres than is that of 
the Frog. Though the heart of the latter animal receives cardio-inhibitory fibres 
through the vagus, their centre of origin is not usually in action, as shown by the fact 
that cutting the vagi does not lead to pulse-quickening ; exactly the reverse and to a 
very marked extent is the case in the Dog (see especially V. Bezold) (5), and also in 
Man as shown by the phenomena observed in cases of atropin poisoning. In addition, 
the Mammalian heart receives from the cerebro-spinal centre accelerator nerve fibres, 
and the existence of any such pulse-quickening fibres in connexion with the Frog’s 
