+Congenital Defects of Bones.+ Various congenital deformities of the limbs occur because of interference in various ways with the proper and normal formation of these cartilaginous masses. If, for any reason, the cause of which in most cases is not clear, any of these cartilaginous masses fail to be formed in the embryonic tissues, naturally no ossification can occur, and in such cases there may be a partial or complete lack of development of the corresponding bone. The amount of this congenital deformity may vary from the absence of an entire foot, to the absence of one or several digits, or one or more phalanges. The deformities produced by such a failure to deposit the cartilaginous base of the bones are very numerous, and in some cases lead to great deformity and loss of function. This lack or increase of the reformation in cartilage, results in most extraordinary deformities. No special type of deformity merits special attention; the condition in each case must be decided by inspection and X-ray examination. In many of these cases, especially where the lesion affects the digits, the capability of the individual is but little impaired, whilst in other cases, where bones are absent, marked deformity and impairment of function may occur. Some of the cases, notably webbed toes, are comparatively easily corrected; other cases however, offer little chance of sufficient cosmetic or functional gain to make a surgical operation necessary or desirable. +Atrophy of Bone.+ Various causes may lead to atrophy of bone. The method by which atrophy is brought about is peculiar, and is due to the action of special giant cells, called osteoclasts. Wherever extensive atrophy of bone takes place, microscopic inspection shows such giant cells lying closely adjacent to the trabeculae of the bone which is being resorbed, and the trabeculae in that immediate vicinity slowly disappear under the action of these giant cells. Their action is very similar to the action of giant cells in the soft tissues about absorbable foreign bodies. This process is called _lacunar resorption_. In old people the amount of absorption oftentimes is very great; the process is then termed _senile atrophy_. It may be marked in the skull and in the long bones, and in many cases of fracture of the neck of the femur, a moderate amount of lacunar resorption precedes the fracture which results from slight violence. In certain cases this resorptive process in old people is extreme, and leads to great fragility of the bones, with repeated fractures from slight violence, which under ordinary circumstances, would cause no injury at all.