CIRCULATORY SYSTEM — PULSATIONS OF THE HEART. 
299 
tioiis from 50 to 98 beats per minute, and a young Helix a.'fpersa from 
70 to 110. wliile if the warmth be increased the heart’s action seems 
to degenerate to a rapid and tremulous agitation, after which heat 
rigour probably soon supervenes. 
Reductions of the temperature will similarly lower the pulse with 
equal rapidity, the pulsations becoming less and less numerous 
until, at a few degrees below freezing point, they may be reduced to 
3 or 4 feeble contractions per minute. A HyaLinia cellar ia, whose 
pulse whilst on the palm of the hand beat at the rate of 74 per minute, 
was exposed to the cool draught at a slightly open window and the 
pulse within two minutes fell to 26. 
The heart, being so responsive to variations of temperature, 
quickening under the influence of noon-day warmth, and being re- 
tarded by the colder air at night, produces, in the rate of pulsations, 
a well-marked diurnal range, which more or less closely corresponds 
to the daily variations exhibited by the thermometer. 
This susceptibility is indeed so delicate that even the play of the 
observer’s warm breath during a short examination is sufficient to 
appreciably accelerate its rate of contraction ; we should, therefore. 
l^iimher of he.'irt pulsations per minute. 
0 5 10 15 20 25 30 35 40 45 50 55 
Fig. 589. — Thermal-pulsatoiy curves of Helix mfcscois^ Helix ^i^ranulala and Hyalinia 
cellaria, illustraiing the efl’ects of fluctuations of temperature as variously aflecting the rapidity of 
the heart’s pulsations in different species. 
naturally expect to find an extremely rapid pulse in summer when 
the thermometer registers 80° or more, but, with few exceptions, our 
species do not voluntarily expose themselves to such heat, but retire 
