Expansion, or rarefaction, though often combined with softening of bone, must yet be distinguished from it. The former is produced by dilatation of the Haversian canals and cells, and constitutes the disease which is named osteoporosis; whilst in the latter there is a deficiency of the mineral constituents of the bone, and some disease of its fundamental cartilage.

Osteoporosis consists, then, as has been said, in an enlargement of the Haversian canals and cells of the bone.

Excessive Development Of The Medulla Of The Bone

a. This state may result from excessive development of the medulla of the bone, or of the tissues which occupy its canals and cells; while, at the same time, the actual quantity of bony substance remains unaltered. By a rarefaction of its tissue of this kind the bone becomes increased in volume, - expanded. The walls of the enlarging cavities become thinner and thinner, till at length apertures are formed in the interior of the bone, as well as in its outermost lamella, and the cavities communicate with one another. The expanded bone is soft, coarsely porous, and spongy, and more or less so in proportion to the degree of the disease; it yields to the pressure of the finger, and may be easily cut with a knife: its cavities are filled with a large quantity of darkish-red or reddish-brown medulla, which is traversed by dilated Vessels, and contains here and there loose or firm clots of extravasated blood.

Osteoporosis may affect the outer compact portion of a bone, and then, in a cylindrical bone, the dilatation of the longitudinal canals gives it the appearance of being split into filaments; or the disease may be developed in the interior in the medullary cavity of a long bone. In the latter case, as the rarefaction advances gradually towards the outer layers of the compact wall, the peripheral lamellar system, though preserving its compact state, becomes distended and bulges all round, the cavity exceeds its natural size, and the bone loses its proper form. The bone is swollen out into a rounded, hollow-sounding, thin-walled cylinder, which is filled with marrow: at its inner wall are found irregularly dilated longitudinal canals, while nothing remains of the spongy tissue and the network in its interior, but a few delicate lamellae and threads of bone, which pass across the plug of marrow that fills the canal; so that after maceration, the medullary canal is a mere dilated cavity. Of course, the bone is uncommonly fragile, as it is in the allied disease, ex-centric atrophy.

Lastly, osteoporosis sometimes affects a bone in its whole thickness; and then the disease may have commenced in its interior in the medullary cavity or diploetic substance, or at the exterior, or at all of these points at once.

This kind of rarefaction usually affects the whole of a single bone, or single portions of the skeleton which are intimately associated together, such as the bones of a limb or of the skull; and it affects such bone or bones throughout; more rarely it is partial, i. e. confined to one spot on a bone; the most unusual instance of all is to find it in several bones together. When it is partial it gives rise to the spongy exostosis.

It is most frequently observed in the skull, and advances in that part to a very great degree, the cranial bones in the adult reaching, and sometimes exceeding, a thickness of six or ten lines. It may occur at any period of life, but it is found chiefly in childhood or old age.

This form of osteoporosis, as has been pointed out, is a consequence of excessive development of the medulla of the bone, and of the soft parts which fill its cavities. No distinct general constitutional disease (dys-crasia), can be assigned as its cause, though it is very important to observe, that osteoporosis, whilst it is one way in which rickets exhibit itself, also frequently recurs in old age, in persons who have other marks of rickets upon them.