512 THE RESPIRATORY EXCHANGE 



skeletal muscles and the muscles of the blood-vessels. During 

 natural sleep, when nervous and muscular activity are at their 

 lowest, the respiratory exchange and temperature fall ; during 

 muscular exercise they rise. The means whereby the power of 

 regulating temperature can be diminished, suspended, or abolished 

 are those which lessen or interrupt the influence of the nervous 

 system upon the muscles, both voluntary and involuntary. The 

 experience of Arctic explorers and Alpine climbers shows that 

 the weary traveller, who gives way to the imperative desire to 

 sleep upon the line of march, is doomed to death. During the 

 deep sleep which follows intense fatigue, the sensibility of the 

 nervous system is so greatly reduced that the response to cold 

 ceases ; the unconsciousness of sleep gradually passes into a condi- 

 tion of paralysis and the man dies, " frozen to death," it is said, 

 but in reality killed long before his temperature falls to zero. The 

 activities of the tissues, once the nervous control is lost, are 

 reduced by the cold ; chemical change and the production of heat 

 become less and less, until an internal temperature about 20 is 

 reached, when life ceases. During such marches the desire to 

 sleep and the effects of cold must be resisted by forced activity 

 and increased production of heat until shelter and artificial heat 

 can be obtained. 



In deep anaesthesia produced by chloroform, ether, or other 

 drugs the activities of the body are so profoundly depressed that 

 the regulation of temperature is suspended ; the production of 

 heat is greatly diminished, and the animal, if it be exposed even 

 to moderate cold, will rapidly cool. An anaesthetised mammal or 

 bird is in many respects comparable to a cold-blooded animal. 

 The influence of the anaesthetic is manifested especially on the 

 central nervous system ; sensations of heat and cold no longer 

 arise, or should they arise no longer produce reflexly decreased 

 or increased activity of the skeletal muscles and the muscles of 

 the blood-vessels ; control is therefore lost over both production 

 and loss of heat. The temperature of anaesthetised animals will 

 fall to 22 when they are exposed to cold, and the cold rather than 

 the drug brings about a fatal issue. The respiratory exchange of 

 an anaesthetised mammal is increased by a rise, diminished by 

 a fall in external temperature. This may be illustrated by the 

 following details of an experiment upon a mouse. 



