The organs of the body do not all start to grow old at the same time or progress at the same time. That such variations in involution may be so exaggerated as to become morbid without any very obvious cause is highly probable, but the latter event is clearly a departure from the progress of normal old age. The precocious atrophy of some tissues or organs may be ascribed to several factors, such as inherent weakness, the effects of overstrain, though without producing gross changes, or to the influence of a definite infection or intoxication in the past. Thus deafness may be hereditary, senile paraplegia has been known to occur in energetic walkers, and thyroid deficiency may be the outcome of a past attack of enteric fever. These errors in the chronometry of life, as Sir James Paget[202] termed the different ageing of organs, cannot be regarded as a physiological process. X DISEASES IN AND OF OLD AGE Strictly speaking, it cannot be said that there are any diseases special to length of days, for premature senility shows the changes and diseases usually correlated with ordinary old age. Inherent want of vitality and the resulting degenerative atrophy, or Gowers’s abiotrophy, may imitate the results of prolonged wear and tear of the tissues, and thus it appears that Charcot’s[203] group of diseases special to old age, namely senile marasmus, senile osteomalacia, senile atrophy of the brain, senile heart weakness, and arteriosclerosis, are not confined to senescence. Old age, however, is prone to the incidence of diseases which are chiefly but not exclusively seen in the evening of life, such as those due to the degenerative changes resulting from the accumulated effect of past infections and from metabolic disturbance. Thus arteriosclerosis, granular kidney, cardiac failure, cerebral haemorrhage, emphysema, hepatic cirrhosis, prostatic enlargement, and carcinoma commonly appear in the sixth decade. In a series of five publications dealing with the diseases of the age of fifty, which he calls the critical age, Leclercq[204] describes, in addition to some of the above, gout and paragouty affections, obesity, diabetes, cardio-aortic diseases, and albuminuria. Old age, moreover, modifies the manifestations and course of infections, notably of pneumonia and erysipelas. It would be unnecessary and from reasons of space impossible to refer to all the diseases that may attack the aged, but a few remarks will be made about some disorders that appear to call for special notice.