236 PRINCIPLES OF PHYSIOLOGY 



phagocytic action of the colourless cells of 

 the blood has been invoked, but there is no 

 clear evidence in support of these views. 

 In old age there is a gradual process of 

 degeneration more or less of all the tissues 

 the nervous tissues suffer least and last. It is 

 not easy to understand why there should be 

 this tendency to degeneration, and conceiv- 

 ably, in perfect hygienic circumstances, and 

 with an absolutely clean pedigree, such de- 

 generation might not occur. Early death 

 should only be caused by accident. In an 

 ideal physiological life, death should come 

 late as a result of gradually increasing weak- 

 ness without pain or suffering, and it should 

 be a process as normal as going to sleep. In 

 most instances, however, degenerative changes 

 do not affect all the organs alike. Some, such 

 as the heart, or lungs, or kidneys, probably 

 on account of an error in the mode of life, or 

 a want of adaptability to the environment, 

 may undergo changes that unfit them for 

 their work. This disturbs the physiological 

 balance and there may be suffering ending 

 in death. When this occurs, the mechanism 



