This extremely simple theory appears perfectly logical, but unfortunately does not fit in with all the observed facts. In reality, rachitis attacks children whose supply of milk, from a chemical point of view, leaves nothing to be desired. The same is true of animals, particularly of young pigs. The so-called “acid theory” has therefore been advanced to explain the points left obscure by its predecessor. =The acid theory.= According to this theory, the food may contain more than sufficient mineral material without, however, preventing the development of rachitis or of osseous cachexia. In animals suffering from digestive disturbance the alimentary tract may become the seat of excessive fermentation or of changes in secretion. There is thus produced an excess of lactic acid which passes into the circulation and accumulates in the tissues, checking the processes which end in ossification or, in the case of adults, even leading to decalcification. It seems fairly well established that experimental administration of lactic acid to animals causes diminution in the quantity of calcium salts contained in the bones (Siedamgrotsky, Hofmeister). On the other hand however Arloing and Tripier failed to produce rachitis experimentally. Bouchard revived this theory in a somewhat modified form. He considers that calcium salts are absorbed as carbonates and chlorides and phosphoric acid as phospho-glyceric acid. The reaction which these compounds undergo within the organism ends in the formation of the phosphate of calcium necessary to ossification, but this “phosphate of ossification” cannot be deposited if the organism contains an excess of lactic acid. =Theory of inflammation.= A third theory which until now has received very little support is that called the theory of inflammation. The general lesions which characterise rachitis are regarded as resulting from primary attacks of ostitis and osteo-periostitis. The cause of these forms of inflammation is not suggested. To the above views may be added that more recently emitted by Dr. Chaumier, according to which rachitis is of an infectious nature. Unfortunately no proof of this has yet been adduced. GENERAL DISEASES. RACHITIS.