462 Mr. J. W. Pickering. [Jan. 26, 



1. Temperature. 



Each embryo has an individual rhythm of its own, which, if the 

 conditions are constant, remains unaltered, but different embryos, 

 even of the same age, may have different rhythms, so that it is neces- 

 sary to determine for each embryo its normal rhythm before varia- 

 tions can be studied. An embryo's heart, aged seventy-two hours, at a 

 temperature of 31 C., was beating with a regular rhythm of 84 per 

 minute. The temperature of the air of the chamber was rapidly 

 raised to 42 C., when the rhythm rose to 91 per minute. A further 

 rise to 50 C. increased the rhythm to 128, it still remaining regular. 

 The temperature was then rapidly lowered to 26 C., when the rhythm 

 fell to 114 per minute. A further fall to 16 C. reduced the rhythm 

 to 34 per minute. The temperature was then raised to 46 C., when 

 the rhythm rose to 117 per minute. On again letting the tempera- 

 ture fall to 25 C., the rhythm fell to 36 per minute. 



The above experiment, taken as an instance from several, shows 

 that, other factors being constant, the rhythm of the embryonic heart 

 varies directly with the temperature of the surrounding medium. 



Extremes of temperature stop the heart ; thus exposure to a tem- 

 perature of 10 C. causes the beats to become weaker and slower, 

 and finally to stop in diastole. If the air of the incubator be raised 

 above 50 C., the beats become so rapid as to be uncountable. They 

 are feeble, and the heart is pale, due to the passage of less blood 

 through it than in the normal state. Violent systolic spasms alternate 

 with periods of quiescence. It stops in an expanded condition when 

 the surrounding temperature is about 55 C. Lowering the tem- 

 perature restores the beating, but the heart is enfeebled. If the 

 temperature is raised much above this limit the heart is killed. 

 Mechanical stimulation of the heart in standstill, due to either 

 extreme of temperature, if applied at the ventricular end, gives rise 

 to one or more waves of contraction, commencing from, the auricular 

 end, and showing the direct conduction through the fibres of the 

 heart. The heart will respond to auricular stimulation when irre- 

 sponsive to ventricular stimulation. Small variations of temperature, 

 such as one or two degrees, occurring over a long period of time, as 

 in an hour, do not affect the rhythm. 



2. The Introduction of Drugs. 



The drugs employed were applied directly to the heart substance 

 at the temperature of the embryo, and dissolved in normal saline 

 (0*65 per cent, sodium chloride) solution. 



a. Ca/eine.An embryo, aged sixty-eight hours, at 33 C. had a 

 rhythm of 88 per minute. To its heart O'OOOIS gram* of caffeine was 

 * All -weights of drugs used are expressed in grams. 



