Atrophy of bones occurs under many forms and various circumstances.

(1.) After a long-continued and exhausting disease of bone, such as caries, after exhaustive healing processes, such as fractures (and injuries generally), or in consequence of palsies, neuralgia, or anchylosis, single bones, or large portions of the skeleton may diminish in volume. They retain their normal texture, but diminish in length, and still more in thickness; an entire bone becomes small, and its medullary canal contracted, - it is in a state of concentric atrophy. And in connection with this fact, and in opposition to that increase in the volume of bones to which I have before adverted as a consequence of distension of the cavity which they form, I may mention that bony cavities diminish, or sink in, when atrophy or loss of substance happens to the organs contained within them.

(2.) Bones are subject in old age to a form of atrophy (senile atrophy), in which their consistence and strength are so far changed that they not only become soft and flexible, and are easily indented, but rather acquire something of the brittleness of glass. (Fragilitas Vitrea. Osteopsathyrosis of Lobstein).

Atrophy appears always to commence with the medullary tubes and diploetic structure: the cells of the latter enlarge, and its walls, as well as the bony threads composing the cancellous tissue, become attenuated, and at length disappear entirely. The compact substance yielding next, is all changed into spongy, diploetic tissue, except its outermost layer, but though that remains compact, it becomes extremely thin, and is sometimes scarcely as thick as a sheet of paper. As the atrophy of this once solid, but now spongy, substance becomes more complete, the outermost layer alone remains, encompassing either a cavity, which contains some mere traces of spongy tissue at its periphery, - a cavity relatively dilated (excentric atrophy), or a soft substance, with very coarse cells; or lastly, when the diploe is entirely removed, the extremely thin remaining parts of the walls of the bone approach each other, and coalescing form a single thin plate. Examples of the first change are presented by the larger medullary tubes; the second is seen in the smaller cylindrical bones, in the pelvic bones, ribs, and vertebrae; while the bones of the face, and small spots, which are limited to the top of the parietal bones of aged persos, exhibit the last. Finally, should the atrophy proceed to an extreme degree, and involve the last remaining thin layer of the wall of the bone, its surface becomes rough and porous; and, however easily the periosteum may be stripped off elsewhere, it cannot be removed from this spot without bringing away a layer of bone with it.

The medullary tubes and dilated cellular interspaces are filled with marrow, which is usually of a dark, and often of a brownish-red, or a chocolate color.

The description given above sufficiently explains how it is that the bones become unusually flexible, and easily crack when they are bent; that upon making moderate pressure on spongy bones the finger breaks into them; that the cylindrical shafts, being reduced to a thin compact wall, break upon the slightest occasions; and that, in advanced age, the well-known curvatures of the vertebral column occur, and other portions of the bony fabric of the body become, under certain circumstances, crooked and deformed.

As the bones lose substance in their interior, they shrink in their external bulk: and hence the skeleton in old age becomes smaller in all its dimensions, and the weight of the body less.

Senile atrophy runs a chronic course, and is unattended with pain; in its earlier stages the muscles also waste and the lungs become atrophied; and afterwards the diminution of the bones is attended with decay (involution) of all the other organs: the muscles are sometimes the seat of fatty degeneration. There are some diseases of bone, "which, in their anatomical characters, present considerable resemblance to this form of atrophy; but, although hitherto thought much of, those characters are in themselves unsatisfactory. They are diseases which affect persons unlike those in whom senile atrophy is met with, and in their symptoms and course they materially differ from it. As they are processes connected with constitutional disease (dyscrasia), and we partly recognize in them, and partly have reason to assume, the existence of important qualitative deviations from a healthy state of the organization, I shall speak further of them under the head of Morbid Expansion and Softening of Bones.

(3.) A third form of atrophy is that in which bone is worn away or absorbed (Usura, detritus ossis). Being occasioned either by uniform and permanent, or by repeatedly renewed (pulsatile), pressure upon the bone, the breach of substance is always circumscribed. Various tumors, which form in the soft parts adjoining a bone, especially in periosteum, produce this effect with different degrees of force: the walls of the skull, for instance, are pressed upon by the Pacchionian bodies, by sundry adventitious growths, commonly known by the collective name of fungus of the dura mater, by tumors in the brain, even by the brain itself, when enlarged or displaced, by large apoplectic cysts, by an enlarged and diseased pituitary gland, etc.; the bones of the face suffer from the pressure of fibrous tumors (fibrous polypi), of sarcoma, and cancerous growths developed in the nostrils, frontal sinuses, antra Highmoriana, or orbits; but the most frequent cause of pressure is aneurismal tumors, and the common seat of it is the bones of the trunk and limbs.

The degree and the extent to which the bone is worn away varies in different cases, the former depending on the amount and duration of the pressure, the latter on the size of the tumor: one or more bones may be entirely destroyed, or a bony wall may be perforated, as is often strikingly illustrated in the progress of aneurismal tumors. As large tumors press unequally, the destruction of bone, when very extensive, is generally not uniform: for the same reason its boundaries are not sharply defined.

Small tumors, which exert uniform and very moderate pressure, and even larger tumors, when they grow slowly, occasion, first, flattening, and then an excavation of the bone on which they press, but do not disturb the smoothness and polish of its surface. The bone immediately around bulges out, and appears not as if it had sustained an actual loss of substance, but rather as if its substance had been merely thrust aside. When pressure is made on one of two compact tables, especially in the skull, it is not so much that table which seems to be absorbed, as the layer of diploe beneath it; the two tables are in this manner gradually brought nearer together, and at length come into contact and unite. I find this borne out by many well-marked cases in the University Museum at Vienna; as well as by most of the pits on the inner surface of the skull, in which Pacchionian bodies have been imbedded.

If the compact wall of a bone be subjected to considerable pressure, it disappears layer by layer, becomes rough on the surface, and when at length entirely absorbed, leaves the cancellous tissue beneath it exposed. A very manifest effort of nature is then often perceived to resist the injury, and to maintain the integrity of the inner texture of the bone.

The cancellous substance, increasing in density by the addition of bony matter to its lamellae and threads, strives to become compact, and exposes to the pressure a stratum as capable of resistance as possible.

This wearing down (detritus) of bone may easily be confounded with the loss of substance which results from caries; and the difficulty of distinguishing between them is sometimes augmented by their both occurring together. For, to take a frequent instance, that of softening malignant tumors, not only is the bone worn away by the pressure which such tumors exert, but inflammation and suppuration are set up in its exposed spongy texture, or corrosion on its surface, by the ichor which they discharge.

Absorption (detritus) is distinguished from caries by the absence of any change of texture, either at the spot itself, or around it; and by there being neither purulent nor sanious product, nor any osteophyte. The tendency to condensation which is exhibited in the substance exposed to pressure furnishes a further distinctive mark of the detritus.

Finally, bone may be absorbed in consequence of pressure from within: the various tumors developed in the spongy substance, and medullary cavity, the fibroid tumors, enchondroma, osteoid growths, sarcoma, cancer, and the dilatations of the capillary system of the vessels, known by the name of teleangiectases or erectile tumors, all commence their ravages within the bone.