232 HUMAN BIOLOGY 



or the other, but that there are successive Hnes of defense 

 set up against the shift. If dilatation of the skin vessels 

 does not stop the rise of body temperature, sweating super- 

 venes; if conservation of heat is not enough to stop the 

 fall of temperature, a second Hne of defense appears in the 

 action of the sympathico-adrenal mechanism, and a third 

 in shivering. Of course extreme conditions can break down 

 these defenses, and a person may die of heat stroke or of 

 freezing. Within a wide range of temperature variations 

 in the external environment, however, we maintain the 

 temperature of the internal environment at an astonishingly 

 uniform level. 



The dedicate control of the body temperature indicates 

 the operation of a sensitive thermostat. The location of 

 this part of the regulatory apparatus is in the base of the 

 brain, in the so-called thalamic region. If the blood going 

 to that region is warmed, the surface vessels are relaxed 

 and sweating takes place; if the blood is cooled, shivering 

 results. When that region is destroyed, regulation is lost 

 and the animal is changed to the cold-blooded type, i.e., 

 its temperature now follows the changes of its surroundings. 

 Anesthetics, such as ether and chloroform, and also excessive 

 amounts of alcohol, have similar effects. In fever the ther- 

 mostat is set for a higher temperature level. 



Adjustments for Maintaining an Adequate Oxygen Supply. 

 The cells of the body are more closely dependent on oxygen 

 than on any other substance obtained from the outer 

 world. We can hve without food for weeks, and without 

 water for days, but there are important nerve cells in the 

 brain which cannot live without oxygen for longer than 

 about eight minutes. These differences appear to be due to 

 differences of storage of these substances in the organism. 

 Food and water are stored, as we have seen, but since 

 oxygen is present all about us, as one-fifth part of the 

 atmosphere, there is no need for storage and to a note- 

 worthy degree there is none. The problem in times of need, 

 therefore, is that of conveying the oxygen to the cells from 

 the surrounding supply. 



Although such need arises after profuse hemorrhage, 

 for example, or in poisoning by illuminating gas or auto- 



