The expression changes, the face becomes more impassive because the facial muscles no longer reflect the constant and ever changing impressions which the youthful sense organs convey to a youthful and active brain.  That the young should ape the old, should seek to acquire the gravity of demeanor, to restrain the quick impulse, is not of advantage.  Loss of weight of the body as a whole is not so apparent, there being a tendency to fat formation owing to the non-use of fat or fat-forming material which is taken into the body.  One of the most evident alterations is a general diminution in the fluid of the tissues, to which is chiefly due the lack of plumpness, the wrinkles of age.  The facial appearance of age is given to an infant when, in consequence of a long-continued diarrhoea, the tissues become drained of fluid.  Every market-man knows that an old animal is not so available for food, the tissues are tougher, more fibrous, not so easily disintegrated by chewing.  This is due to a relative increase in the connective tissue which binds all parts together and is represented in the white fibres of meat.

Senile atrophy is complex in its causes and modes of production.  The atrophy affects different organs in different degree and shows great variation in situation, in degree and in progress.  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.  An infection which may but slightly affect the ordinary well-being of the individual may produce a considerable damage.  Excess or deficiency or improper food, occasional or continued use of alcohol and other poisons may lead to very definite lesions.  Repair after injury is rarely perfect, the repaired tissue is more susceptible to injury, and with advancing age there is constant diminution in the ease and perfection of repair.  The effect of the sum of all these changes becomes operative:  a vicious circle is established in which injury becomes progressively easier to acquire and repair constantly less perfect.  There is some adjustment, however, in that the range of activities is diminished, the environment becomes narrower and the organism adapts its life to that environment which makes the least demands upon it.