It is well known that progressive muscular debility is an accompaniment of old age. Physical work is seldom given to men over sixty years of age, as it is notorious that they are less capable of it. Their muscular movements are feebler and soon bring on fatigue; their actions are slow and painful. Even old men whose mental vigour is unimpaired admit their muscular weakness. The physical correlate of this condition is an actual atrophy of the muscles, and has for long been known to observers. More than half a century ago, Kölliker,[15] one of the founders of histology, devoted some attention to this matter, and described the senile modification of muscular tissue in the following words:—“In old age there is a true atrophy of the muscles. The fibres are much more slender; there are deposited in their substance numerous yellow or brown granules and many globular nuclei. These nuclei are frequently arranged in longitudinal series and present such signs of active division as are found in embryonic tissue.” Other investigators afterwards made similar observations. Vulpian[16] and Douaud[17] have stated that a multiplication of nuclei takes places in the atrophying muscles of the old. As the senile degeneration of muscular tissue appeared to be important in my study of the mechanism of senescence, M. Weinberg and I examined several cases of muscular atrophy in old human beings and lower animals. We were able to recognise the phenomena observed by our predecessors. In senile atrophy the muscular fibres contain many nuclei, and these, increasing rapidly, bring about an almost complete disappearance of the contractile substance (Fig. 9). The fibres preserve their striation for a certain time but eventually lose it and appear to contain an amorphous mass with numerous, rapidly multiplying nuclei. [Illustration: FIG. 9.—Degeneration of striated muscle Fibres from the auricular muscle of a man aged 87 years. (From a preparation made by Dr. Weinberg.)]