70 THE EVOLUTION OF MAN 



the curves of the female, the waist is broader, the pelvis 



smaller, the shoulders less sloping, and the hands and feet 



larger. 



SENESCENCE 



The normal life cycle has three phases, development, ma- 

 turity, and decline. Each of the first two phases lasts about 

 thirty years and the third should be at least of equal length. 

 But this may be greatly exceeded as shown, for example, by 

 the longest life yet recorded in modern times; that of Thomas 

 Parr of England who was one hundred fifty-two years and nine 

 months old when he died. Old age in itself is a physiological 

 condition which results from a state of lessened vitality and 

 activity of the various organs. The essential factor in grow- 

 ing old is a progressive degeneration of the tissues, as a result 

 of changes in the cells which compose them, thereby bringing 

 about a gradually diminishing functional capacity of the 

 various organs until, finally, the activity of some vital organ or 

 organs ceases and physiological death results. Certain visible 

 external structural changes in the body are increasingly in 

 evidence as old age comes on, all of which are due funda- 

 mentally to degenerative changes in the individual cells of the 

 various tissues. The hair turns gray, the skin loses its elas- 

 ticity and becomes wrinkled. Extensive changes occur in the 

 jaws which are due to a loss of the teeth, with a consequent 

 absorption of that part of the jaw which held them, resulting 

 in a return to the infantile type of jaw. The walls of the 

 arteries harden (arteriosclerosis) and the blood pressure in- 

 creases. There is a progressive loss of strength and elasticity 

 in the muscles. Some of the cartilages may calcify, the bones 

 become more fragile, the cranial sutures disappear, and there 

 is a decrease in the stature. The near point of vision recedes, 

 due to a loss in elasticity of the mechanism of accommodation. 

 Changes also take place in the nervous system. These are 



