_The prostate_ shows some degrees of enlargement after the age of fifty in the vast majority of men, but in only a percentage of these are there symptoms referable to it. Kenneth Walker[170] finds that the maximum size is reached at the age of 60 and that from then onwards there is a slow diminution in size; among 340 men between 80 and 90 there were 11, or 3·2 per cent, and among 92 men between 90 and 100 one only with hypertrophy of the prostate (Humphry). The causation of prostatic hypertrophy has been much discussed; that its association with arteriosclerosis (Launois[171]) is anything more than a coincidence, the two conditions being common in the later years of life, seems improbable; Walker found the two associated in 10 per cent, and he regards the change as part of a general enlargement and thickening of the peri-urethral, sub-cervical, and sub-trigonal glands, and, as the interstitial cells in the testes become fewer and degenerated, he considers that the prostatic enlargement is possibly a degeneration connected with a disturbance of the endocrine balance. Nemenow[172] argued that prostatic enlargement was due to proliferation of the interstitial cells following senile atrophy of the seminal tubules of the testes, but K. Walker found that in prostatic enlargement the interstitial cells are diminished rather than increased in number. An interesting parallel has been drawn between the involutionary changes in the mamma and the prostate, and it is probable that the same underlying factor is at work in both (Walker, Paul). Hertoghe[173] regarded some cases of prostatic hypertrophy as due to senile dysthyroidism, and recently benefit has been reported from thyroid medication and also from prostatic extract. Dr. Leonard Williams has told me of cases, as yet unpublished, showing well-marked relief of symptoms and diminution in the size of prostatic enlargement after doses of thyroid extract (½ grain once) and colloidal iodine (one dram three times) daily. The prostatic plexus of veins is often enlarged and may contain phleboliths. The _testes_ become smaller, softer, and commonly show some atrophy of the tubules with disappearance of the epithelial lining and thickening of the basement membrane; but the testes of old men may be free from any such change and the spermatozoa in the vesiculae seminales may be active. According to K. Walker the interstitial cells gradually diminish in number from the age of 30, but they may be present in men over 80, and Mott[174] remarks that their persistence may account for an increased and perverted sexual appetite, due to stimulation of the desire without the power to perform the sexual act. The _penis_ becomes smaller, often retracted, the _glans_ harder, and the _scrotum_ smaller.