40 BIOLOGY: GENERAL AND MEDICAL 



maintain a stable body temperature through heat- 

 regulating mechanisms, by which the heat resulting 

 from the metabolic processes is prevented from radiating 

 when the external temperature is low, or radiated rapidly 

 when it is high. For these organisms any considerable 

 variation in the temperature of the body itself is quickly 

 fatal; so that, when they are unable to prevent loss of 

 heat by radiation, through lack of proper protection in 

 the way of hair or feathers, they quickly die of cold; or, 

 if through any means they are prevented from radiating 

 heat, they quickly die with an elevated body tempera- 

 ture resulting from the accumulation of heat. 



The eggs of birds which have no means of maintaining 

 or radiating heat, are affected by slight variations of 

 temperature, and afford striking examples of the stimu- 

 lating as well as the destructive effects of temperature. 

 Thus, if a hen's egg be placed in an incubator under favor- 

 able conditions, the irritability of the germinal cell is 

 shown at about 39 C. by division and a succession of 

 changes that will eventually result in the development 

 of a chick. If, however, the incubator cool, or if its 

 temperature rise a few degrees and remain so for a few 

 hours, development ceases and the embryo dies. 



When we come to consider man we find a high degree 

 of temperature susceptibility. Normally, the body 

 temperature is 37 C. and at this point it is maintained 

 by complicated heat-regulating nervous mechanisms, in 

 spite of external conditions. His cells are, however, 

 so susceptible to changes of temperature in the body 

 itself that a variation of more than one degree cannot 

 take place without subjective symptoms; a variation of 

 more than two and one-half degrees, without subjective 

 and objective symptoms and incapacitation from the 

 usual activities of life; a variation of three degrees with- 

 out prostration, or of five degrees without danger to life. 



It is the thermal irritability of protoplasm that leads 

 to the varying vital manifestations accompanying the 

 procession of the seasons as it is seen in the temperate 



