Atrophic changes of the blood vessels are of great importance, for this affects the circulation on which the nutrition of all tissues depends.

While there is undoubted progressive wear of all tissues, this becomes most evident in the case of the blood vessels of the body.

It is rare that arteries which can be regarded as in all respects normal are found in individuals over forty, and these changes progress rapidly with advancing age.

So striking and constant are these vascular changes that they seem almost in themselves sufficient to explain the senile changes, and this has been frequently expressed in the remark that age is determined not by years, but by the condition of the arteries.
Comparative studies show the falsity of this view, for animals which are but little or not at all subject to arterial disease show senile changes of much the same character as those found in man.
There is another condition which must be considered in a study of causes of age.

In the ordinary course of life slight injuries are constantly being received and more or less perfectly repaired.