Many explanations are offered for the nature and pathogenesis of rachitic bone processes. The decalcified remains of the new formed bone tissue finally reach the stage at which they are unable to absorb the lime salts that are abundantly present for their use. Only in this one respect is the calcium metabolism disturbed by rachitis; beyond a certain point the question is as little one of deficient calcium supply as it is of defi cient calcium absorption.
It is true that if an experimental diet very poor in calcium is given to quickly growing young animals, it leads to a disease which seems, upon superficial examination, to be extraordinarily similar to rachitis. Histological examination, however, shows that here we have to deal not with true rachitis, but with an osteoporosis with rachitic-like changes in the periosteum and in the growing cartilages. With this pseudo rachitic osteoporosis, in contrast to rachitis, the soft tissues also take part in the calcium deficiency. Furthermore Pfaundler has very re cently proved in a brilliant manner that rachitic osteoid tissue is not able to correct its calcium deficiency by means of absorption from a solution of calcium chloride, but that in the pseudorachitic osteoporosis of animals fed with a diet deficient in calcium the calcium absorption is markedly increased.
Rachitis can also be experimentally produced by the feeding of acids in the same way as with a diet deficient in calcium.
The hypotheses according to which rachitis is due to a lessening of the alkalinity of the blood, to the accumulation of lactic acid in the organism, or to the retention of abnormally large amounts of carbonic acid, are as equally erroneous as the one last considered, which assumes a primary disturbance in the calcium metabolism.
In the majority of cases rachitis develops in the colder part of the year in which the children very seldom go out of doors, just as it never occurs in wild animals in their free state, and very seldom in pastured animals, but very frequently in animals in stables and zoological gar dens. Perhaps furthu investigations into these observations may some day lead to the elucidation of the origin of this curious disease.
Diagnosis.—Before the appearance of the characteristic skeletal changes the diagnosis of rachitis can be only presumptively made. In the newborn there occurs not infrequently a defective ossification of the flat cranial bones which can be confused with craniotabes, but it may be distinguished from the latter by the less marked limitation of the depressible areas to the occipital bones, as well as by its course, as it reaches its full development in the first week or month after birth, while true craniotabes never occurs congenitally and usually appears first after the end of the third month of life. Furthermore, according
to the investigations of Friedlehen, there exists in all children during the second three months of life a physiological increase of the bony absorp tion in the flat cranial bones which can be the more easily mistaken for craniotabes as it affects principally the posterior part of the skull. It is more frequent in artificially fed children than in those fed from the breast, and is clinically manifested by the fact that the posterior part of the cranial vault becomes flexible and compressible ; on careful palpation, however, one always finds a feeling of complete elastic ity of the bony plates while in rachitic craniotabes the affected areas have more or less completely lost their elasticity; they are not flexible but soft. Without other symptoms a diagnosis of rachitis cannot be made from the time of the appearance of the first teeth, for in non-rachitic children they often appear first in the 9th or even in the lith month.
The softening of the thorax is shown by the retraction at each inspiration, and is most distinct with quiet breathing, but one should remember that in young children without rachitis the thorax is far more elastic than in later years, and that a corresponding suggestion of inspiratory retraction occurs even in absolutely normal infants. For an early diagnosis the occurrence of the rachitic rosary is far more im portant than the pliability of the thorax. Still it is to be emphasized that slight swelling at the costal end of the costal cartilages is frequently present even without rachitis. Naturally no sharp distinction can be made between these slight enlargements and the somewhat larger ones which are referable to rachitis. The same thing holds good for the epiphyses of the long bones; here too there is a gradual transition between the pathological and physiological enlargements.