OF TEMPERATURE UPON THE BEAT OF THE DOG’S HEART. 
665 
heart is at present doubtful. Consequently, bearing in mind that greater division of 
physiological duties which characterises the higher animal, we may justifiably doubt 
whether the simple relation of higher temperature (within limits) and quicker pulse 
found in the Frog and dependent only on the properties of the heart itself, may not 
be entirely absent in the isolated heart of the higher animal, which we know to have 
its rate of beat under normal circumstances controlled by a highly specialised set of 
extrinsic nerve centres. 
The above considerations, taken in connexion with the fact that “fever” can 
hardly be said to exist in an animal with so variable a normal temperature as the 
Frog exhibits, made it very desirable to study directly the influence of temperature 
variations upon the pulse-rate of the Mammalian heart. 
The experiments hitherto made upon Mammalia do not really solve the question 
whether the quicker pulse of the warmer animal is due to a direct or an indirect 
action (i.e., one exerted by extrinsic nerve centres) upon the heart. Bernard, Walther, 
Horwath, and no doubt others have found a slow pulse in artificially cooled animals ; 
the same phenomenon has been observed in hybernating Mammals during their 
winter sleep. As regards the effect of heightened temperature upon the pulse-rate, 
Brunton (6) has showed that when Rabbits are heated the heart beats quicker. 
But when a whole animal is warmed or cooled we are not justified in concluding that 
because the heart beats quicker or slower therefore the temperature change has 
directly influenced the rhythm of that organ. Not only may temperature changes 
indirectly affect the heart through its extrinsic nerves, but they may also so alter 
tissue metamorphosis in various organs as to essentially modify the composition of the 
blood flowing through the heart; and we know that very slight alterations in the 
chemical composition of that liquid may profoundly influence the heart. Before we 
are entitled to state positively that changes of temperature directly influence the 
rhythm of the heart of the warm-blooded animal, we must have data based on 
experiments made with the hearts of such animals cut off from all possible control 
i through extrinsic nerve centres, and supplied with nutriment of constant composition. 
The only experiments known to me which approach the fulfilment of such conditions 
are those made by several observers (Schenk (7), Wernicke (8), Cleland (9)) on the 
influence of temperatm’e changes upon the ra.te of pulsation of the hearts of embryo 
Chicks during the first three days of incubation. Such experiments afford, however, 
even a less safe ground for conclusions as to the adult Mammalian heart than do the 
experiments upon Frogs’ hearts above referred to. The heart of three-day Chick 
embryos is but a protoplasmic mass, little differentiated, presenting neither definite 
muscular or nervous tissue, and without any developed controlling extrinsic nerves. 
From the fact that such a mass of hardly-differentiated embryonic cells contracts more 
frequently when warm than when cold, we cannot safely conclude that the adult heart, 
with its fully developed muscular and nervous tissues, and placed under the 
