Although the immediate causes of the precipitation of the calcium salts must be expressed somewhat vaguely, the places and effects of their accumulation are sufficiently well known, as are the resulting appearances. The presence of these salts in sufficient quantity produces a homogeneous, granular, strongly refractive appearance of the cell or intercellular substance, in addition to a greatly increased resistance to pressure. When muriatic acid is added to the affected part, the salts are dissolved, with the escape of abundant bubbles of gas when a carbonate is present, and with a rapid fading of the glistening appearance, without effervescence, {88} when the salt is a phosphate. After the removal, the cell or intercellular substance is readily recognized, with such modifications in its appearance as may be due to the action of the strong acid. The parts in which this deposition or infiltration has taken place are either relatively normal in appearance or variously altered from disease, and the calcium salts are to be regarded as absorbed from the constituents of the food and deposited, or as taken up and transferred from the bones of the body. That both sources are drawn upon is obvious from the abnormal presence of calcareous material in the soft parts, in connection with increased density of the bones, as well as with a diminution in the density of the latter. The term calcification is more correctly applied to the presence of the salts in normal tissues other than bone, or in the products of disease not simulating bone-cartilage in structure. A pathological ossification is to be considered present when an actual new formation of bone has taken place so limited and so situated as not to suggest a tumor of bone, or when the calcium salts are deposited in a new-formed tissue whose structure stimulates that of bone-cartilage. Tissues which may become calcified are, in the first instance, the connective tissues, and of these fibrous tissue and cartilage are especially liable. Epithelial, muscle--in particular the unstriped variety--and ganglion-cells may also become calcified. The frequency with which blood-vessels, especially arteries, are affected is such that it is regarded as almost normal in advancing years that calcareous material should be deposited within the vascular walls. A distinction is drawn between an ossification and a calcification of the blood-vessels. The former term should be limited to the osteoid plates so often found as circumscribed thickenings of the aortic intima, and which are obviously new-formed patches of fibrous tissue in which the calcium salts are accumulated. A calcified artery, on the contrary, is one usually of a size varying between that of the common iliac and the temporal arteries, whose wall has become rigid and unyielding, suggestive of a pipe-stem, from the presence of calcareous deposits in the muscular middle coat.