HISTORICAL REVIEW 
intervals in a 1- to 4-hour test. The presence or absence of shiv- 
ering is determined visually or by palpation. A normal animal 
will maintain a rectal temperature within narrow limits of 0.5 to 
1.0 C, whereas a poikilotherm will have a continuously falling 
rectal temperature. Experiments which impair temperature regu- 
lation will have a temperature-time record intermediate between 
these extremes. 
Measurement of oxygen consumption rate. This measurement 
is perhaps the most useful and quantitative method of measuring 
shivering and nonshivering thermogenesis. An animal tested for 
shivering is first placed in a comfortable environment of 25 to 
30 C, and after a time interval sufficient to achieve a steady 
state, its oxygen consumption rate (designated as VO ) is meas- 
ured. The oxygen consumption rate is usually expressed as milli- 
liters O STPD per kilogram of body weight per minute. The 
animal is next placed in a cold environment of to 10 C, and the 
oxygen consumption rate continuously measured; the oxygen con- 
sumption rate will rise and finally reach a fluctuating steady state 
value. The fluctuation is caused by the irregularity in shivering. 
The ratio VO (cold environment)/ VO (comfortable environment) 
is a usetul index of the intensity of shivering. Oxygen consumption 
rate can be measured by one of several methods. A convenient 
method used in our laboratory has been to measure continuously 
the O content and the CO content of the gas within a sealed box 
in which the animal is placed. Carbon dioxide and water are re- 
moved by circulation of the gas through absorbing columns. 
Mechanical record of shivering . Since a shivering animal pro- 
duces a well-defined tremor, the presence of shivering can be 
detected and can be roughly measured quantitatively by measuring 
the vibration of shivering. A resting animal is placed on a platform 
which is suspended by wires or supported on springs. The shiv- 
ering causes a vibration of the platform and can be measured 
with a piezo-electric crystal, a sensitive pressure transducer, a 
strain gauge, or more simply with a thread which has one end 
attached to the platform and the other end to a lever arranged for 
kymograph recording. With kymograph recording, however, the 
frequency characteristics of the system are likely to be poor. 
