The disturbances which endochondral ossification undergoes in rachitis are very characteristic. As a result of the delay in the ossifi cation caused by the advancing growth of the cartilage cells, the pri mary growing margin assumes an abnormal width. La-ter on provisional cartilaginous calcification occurs but still only in an incomplete man ner; then, through further cartilaginous growth, the formation of the marrow cavity advances irregularly over the line of ossification, and finally, by a continuation of the above phenomena, narrow marrow cavities arc formed in the cartilage, the edges of which undergo a meta plasia into osteoid tissue. In all stages the newly formed bony tissue remains more or less completely uncalcified in the neighborhood of the line of ossification as it does elsewhere in the skeleton. With a continua tion of the disease these phenomena always become more striking.
Very frequently the rachitic bones show a more or less marked degree of osteoporosis, in addition to the changes which have been above described. The consistency of the bones is probably impaired rather by a marked osteoporoSis than by the persistence of the newly formed bone substance in an osteoid condition. After the complete cessation of the rachitic process the bones attain an abnormal thickness with a correspondingly increased hardness, and since they are thicker than normal there results a considerable increase in their weight. The apophyses and the other muscle attachments are unusually strongly marked, time normally sharp edges are rounded off, and their whole appearance is coarse and unsymmetrical. The erosions on the teeth remain throughout life.
Of the lesions in the soft tissues only the rather frequent occurrence of extensive enlargements of the ventricles of the brain needs mention. Pathological water content of rachitic bones is higher than normal; in the fat content there are no constant differences. The most important chemical property of rachitic bones is the decrease of mineral constituents, especially calcium and phosphoric acid. The percentage of ash in the ribs and vertebre is especially low, often only 25 per cent., or less, of the normal. The specific gravity of the cartilages and the bones is decreased and the relative weight of the cortex of the spongiosa and of the cartilages is changed in favor of the cartilage. The soft tissues of rachitic children contain no less calcium than normal. The urine is usually faintly acid; during the active stage of the disease its calcium content is usually somewhat reduced while the percentage of calcium in the feces at this time is always slightly increased.
Relation of Rachitis to Osteomalacia; Late Rachitis.—The causes of rachitis and of osteomalacia are without doubt distinct. The patholog ical processes in the bone are, however, in a manner the same. In osteo malacia the disturbance of endochondral ossification is le',s marked than the occurrence of osteoporosis, but this difference is explained by the diversity in the ages of the patients. The marked osteoid deposits in the bones in osteomalacia consist of new formed uncalcified bone tissue; in rachitis, on the contrary, there occurs in restricted areas supplemen tary decalcification of already normally calcified bone. Osteomalacia, therefore, from a pathological,—not an etiological,—standpoint, may be considered as a rachitis of later life. The transition forms which occur in young adults, over the classification of which as rachitis or osteo malacia there has heretofore been much argument, present no difficul ties with this perception of their relation. In the cases which have been described as infantile osteomalacia, and which curiously always occurred in girls, rachitis resembles remarkedly in its skeletal phenomena the otherwise characteristic behavior of osteomalacia of later life. These cases of rachitis are in marked contrast to those in which disturbances of cartilaginous ossification are the most prominent manifestations of the disease (von Recklinghausen's "pure rachitis").
According to the opinion of authoritative surgeons the static de formities which appear at the time of puberty develop upon the founda tion of true rachitis. There are in such cases specific raehitic changes in the epiphyseal cartilages, and to a greater or less extent, periosteal osteophytic formation is also found.
Etiology and Pathogenesis.—The cause of rachitis is unknown. Siebert has again recently drawn emphatic attention to the occurrence of hereditary predisposition to the affection which, according to his theory, is transmitted principally by the mother. Children with this tendency are not protected by maternal nursing but in such cases the disease is usually mild. Only exceptionally does severe rachitis occur in breast-fed children. If this hereditary tendency is absent children often remain free from the disease even if fed artificially. Rachitis has no relation to hereditary syphilis.