Mollities Ossium (Osteomalacia, Malakosteon, Rhachitismus adulto-rum, and senilis), is quite a different disease from true rickets, and affects grown persons in the period between early manhood and old age. It occurs chiefly in the bones of the trunk, to one portion of which it so far confines itself as to proceed to a very great degree in that portion, whilst mere traces of it only are found in other bones. When the bones of the skull and of the extremities are affected, they are so always in a very subordinate degree. It is more frequent in the female sex than in the male; and several times it has been met with coming on after childbed. Not unfrequently it is associated with cancer of the internal organs (a fact which reminds us of the old observation as to the brittleness of the bones in cancer). Sometimes it exists when there is a great production of fat, especially in advanced life: and it is often found when there is also fatty degeneration of the muscles: the import of this last combination is not yet understood, whether it is occasioned by insufficient action of the muscle, or has any essential connection with, and is produced by, the general disease. Compared with rickets, and considering how rarely the disease occurs, its advance to a very considerable degree may be said to be frequent.

The deformities which result from mollities ossium are restricted to the trunk, as has been mentioned above. They take place upon the bed to which the patients are confined, and it is this mode of origin that determines the peculiar shape which results from the disease, and which in the pelvis is regarded as characteristic. The two ends of the trunk approach each other by the vertebral column arching backwards; the thorax sinks in, especially at the sides, the ribs becoming curved and bent in various ways; and the pelvis acquires a triangular form, like that of the heart on cards. But these are not the invariable shapes; and the peculiar form of the pelvis is not exclusively a result of mollities ossium, but is met with sometimes in bedridden persons, who are the subjects of rickets in a high degree.

The bones diminish in size, and their texture is rarefied and atrophied; they become saturated with fat, and reduced to their cartilaginous element. In this condition their corpuscles are empty, and when viewed by transmitted light, diaphanous: there are no canaliculi (kalk-kanalchen), and the lamellar structure is lost. The bone at the same time undergoes a striking change in its chemical composition, the extract produced by boiling being not only different from chondrin, but also from the animal matter of bone.

Upon this last-mentioned character of mollities ossium very probably depends not only an essential difference between it and rickets, but also its malignancy: it is a very painful disease, and hitherto has never been cured.