=Pathology.=--Rickets is generally referred to as “fetal” or “congenital,” according to whether the infant presents characteristic markings at birth or whether they develop later. The most marked constitutional defect seems to be in the supply of calcium salts, which leads apparently to formation of bone which has not sufficient compact tissue to make it strong. Especially along the line of junction between bone and cartilage do we see the most marked expressions of rachitic lesions. Here the cartilage is evidently actively growing, while the bone formation proceeds with difficulty, and the proportion of vascular tissue is excessive. The result is prolongations of soft vascular into the cartilaginous tissue, by which the latter becomes more or less absorbed, and this essentially interferes with _ossification_. In severe cases it may be lacking. At epiphyseal lines one may see a layer of osteoid tissue which is not cartilage and will not become bone. Because of its yielding nature it warps under the mechanical strain to which the bones of the extremities in young children are constantly subjected. The obscure but unmistakable relations existing _between rickets and the status lymphaticus_ will be referred to in Chapter XIV. The osseous lesions of rickets differ from those seen in osteomalacia, since in the latter the softened tissue is practically decalcified bone, while in the former case most of the affected tissue has never gone so far as genuine bone formation, but is arrested in its perverted state. The result of rickety changes in the skeleton is a thickening of the shafts of the long bones, of the outer table of flat ones, of the epiphyseal extremities of shafts, and frequently a stunting of their development, so that they do not attain their normal length. The periosteum is also affected in rickets, with the result that when the changes occur, mostly subperiosteally, there are warpings and curvings of the bone shafts, while so long as the disturbance is epiphyseal more or less abrupt curvatures and angular deformities will be produced as the result of muscle action. So marked are the changes in some instances that it has been stated that bones may even lose three-fourths of their calcium salts. When rachitic bones are so soft as to be easily cut with a knife, marked deformities occur as the result of muscular activity. (See Plate XI.)