286 Harris M. Benedict 



Hardening and thickening of the lining of vessels, lungs, and digestive 

 system, and hardening of cartilage. — Arterial sclerosis has long been asso- 

 ciated with senility, even to the extent of being considered a cause 

 (Demange, 1886). The hardening and thickening of all lining membranes 

 is believed to accompany old age, but the experiments have been too few 

 to decide the matter. There seems little question, however, but that 

 such absorbing and excreting surfaces become more rigid with age, and 

 probably less efficient. The hardening of the cartilages, with consequent 

 decrease in the flexibility of the joints and the forward curving of the 

 spine, is universal in man. 



Increasing weakness and hrittleness of hone. — While other tissues are 

 gaining an undue amount of calcareous material, the strengthening lamellse 

 of the interior parts of the bone are disappearing and in consequence 

 the bone becomes weaker. The remainder of the bone increases in its 

 calcareous content and decreases in its living substance, thus becoming 

 more brittle as well as weaker. 



Tendency of connective tissue to encroach on the higher tissues. — The 

 increasing tendency in old age for injured cells of nerves, muscles, or 

 other highly specialized tissues to be replaced by connective tissue, which 

 is the least highly specialized tissue, has been explained in a number 

 of ways. The generally accepted view is that given by Hertwig (1906). 

 He pictures a single-celled organism as carrying on the functions necessary 

 to itself alone, and thus being acted on only by cytological laws. The 

 cell in a multicellular organism, on the other hand, is subject to both 

 the cytological laws required for its own nourishment and to the organo- 

 typical laws governing those of its activities that nerve the whole organism 

 of which it is a part. Hertwig believes that the higher the differentiation 

 of a tissue, the more complete is its obedience to the organotypical laws. 

 As age increases, the organotypical laws weaken, and the lowest tissue, 

 which is least subject to them, is the first to throw off the yoke and 

 attack the surrounding organs. This view is interesting, but purely 

 theoretical. 



While the discussion of these four classes of change in organs seems 

 a very inadequate treatment of the effect of senility on animal organs, 

 it is not an unfair expression of the present state of- knowledge of these 

 changes. Other changes have been mentioned, but there is even less 

 evidence for them than for those here given. 



