Rachitis is a disease of youth, and is common both to the human species and to all domestic animals. It is characterised by irregularities in development and by imperfect consolidation of the bones. The boundary between rachitis and osseous cachexia is difficult to define and in fact at the present moment the two diseases can scarcely be defined with exactitude. Rachitis again is often complicated with softening of the bones, disease of the limbs, arrested development, etc., but it must not be forgotten that although the irregularities in ossification and development of the skeleton are the symptoms most striking to the eye, they do not stand alone, and that from the point of view of development all the tissues, including the muscles, are more or less affected and that most of the physiological functions such as digestion and the secretion of urine are deranged. =Etiology.= One of the principal causes suggested is that of heredity, and so far as human beings are concerned, one seldom fails to discover the rachitic taint. Certainly the offspring of individuals marked by any debilitating disease like alcoholism, tuberculosis, syphilis, etc., are poorly equipped for their future development. Their tissues lack the necessary qualities and, _cæteris paribus_, their physiological functions are performed less perfectly than are those of normal individuals. It is difficult to apply such information to domestic animals, because badly developed subjects are not used for reproduction and the importance assigned to heredity can therefore scarcely be sustained. The conditions of life, on the contrary, have an unquestionable influence, and if rachitis is so frequent in young animals living near towns, for example, it is undoubtedly due to that want of air, light and liberty, which first affects the mother’s health and later that of her offspring. The same may be said of insufficient and improper food; for in this connection quality is of even greater importance than quantity. Even free feeding is insufficient if the fodder does not contain the material necessary for sustaining and building up the developing frame, a point which readily explains the occurrence of rachitis when young animals receive a diet deficient in certain chemical constituents. This occurs in young lambs and pigs where the mothers are given too little variety or too small a quantity of food. In calves and foals rachitis is rare but occurs when the mothers are exhausted or cachectic or are debilitated by chronic wasting diseases like tuberculosis or osseous cachexia. The milk is then no longer of normal chemical constitution.