It is common knowledge that the flesh of old animals, used as food, is tough. An old fowl cannot be compared with a tender and juicy chicken. Organs such as the liver and kidneys are much harder in the case of old animals. The horny flesh of old animals is often compared with boot-leather. Although the comparison does not pretend to be scientific, it is far from being incorrect. Boot-leather is made from the hides of animals; that is to say, of a very resistant material that is called “connective tissue,” and which consists of a dense mass of fibres, mingled with the living elements or “connective tissue” cells. This tissue is very durable and so is employed for boots and shoes. The infiltration of any organ with connective tissue makes it tough and unpalatable. This hardening is called a _sclerosis_ (of the liver, kidneys, &c.). In old age many organs exhibit this tendency to hardening or sclerotic degeneration. The fact has been known for long, but its significance has been perceived more recently. Demange,[293] in his monograph on the organic changes associated with old age, states as follows: “Besides atrophy and degeneration of the parenchymatous elements,[294] there is to be observed a profound change in the framework of connective tissue, which serves to support the organs. In some cases the skeletal framework of an organ becomes more conspicuous, simply on account of the degeneration of the cells; this is the condition usually present in the liver of aged persons. More often, however, the connective tissue receives some kind of stimulation, which, although it does not amount to inflammation, brings about an active growth and resulting sclerosis. According to the particular case, the hardening occurs in the form of isolated patches or strands, or affects the whole periphery or even the depths of the organ, and smothers the higher elements in its meshes, so producing a further degeneration. The cellular elements disappear gradually, connective tissue taking their place, and the change may be so profound, that as in the case of the prostate gland, the altered organ may actually transcend the normal size, partial or general atrophy, however, being more often the result.” Sclerosis in old people sometimes takes the form of a hardening of the liver (cirrhosis of the liver) or of the kidneys (renal cirrhosis), but it is the arteries which are most commonly affected by it, producing a symptom of degeneration which is called arterial sclerosis.