by indefinitely arranged layers of bone, called the interstitial or ground lamellæ. Besides the Haversian canals there is another system of blood-channels. These occur scattered through the circumferential lamellæ, and have received the name of Volkmann's canals. Their blood-vessels communicate both with the surface of the bone and with the Haversian canals. There is no special arrangement of the bone around the Volkmann's canals, as occurs around the Haversian systems, the former being simply spaces between the lamellæ.

Scattered between the individual lamellæ of all three types we find many elongated, thin spaces-the cellspaces, or lacunæ. In the circumferential lamellæ their long axes correspond to the long axis of the bone. In the Haversian lamellæ the lacunæ are slightly curved laterally, to conform to the general concentric annular arrangement. In the interstitial lamellæ there seems to be no special arrangement, but in a general way the long axes of the lacunæ correspond to that of the bone.

Radiating in all directions and connecting the lacunæ are minute canals, called canaliculi, which serve to transmit nutriment to the cells within the lacunæ and communicate with the lymphatics in the canals. While the canaliculi radiating from the lacunæ of a Haversian system intercommunicate very freely, they rarely or never have any direct connection with the adjacent systems.

In dry preparations of bone which have been ground to the requisite degree of thinness for microscopic examination the lacunæ and canaliculi become filled with particles of dirt and debris and appear black.

In sections which have been decalcified, however, the lacunæ are found to contain cells with long protoplasmic prolongations reaching into the canaliculi. These are

the true bone-cells, or bone-corpuscles. If some of the outer lamellæ be forcibly torn off, there will be observed occasional fibers which penetrate the bone vertically to the surface. These fibers are the remains. of old periosteum which in the process of bone formation have failed to be displaced by the newly deposited subperiosteal bone. They are called perforating fibers of Sharpey.

Spongy or cancellated bone differs from the compact in that the plates or spicules of which it is composed have no regular arrangement into a system, but unite irregularly to form a honeycombed mass, the cavities of which are filled with marrow. The nutrition

of the bone is derived from this marrow, hence there is no need for special blood-vessels. Spongy bone is found at the ends of the shafts of long bones and at the centers of the short and flat bones (the apophyses and diploe). At this place it might be well to call attention to several points in which bone and cartilage are analogous-viz.:

1. They both have a common mesoblastic origin.

2. Both are covered by similar membranes-perichondrium, periosteum.

3. Both are composed of cells embedded in an indifferent matrix.

4. Both have similar functions-supportive framework.

MARROW.

There are two kinds of marrow: the red, which occurs in spongy bone,—principally in the flat bones, the vertebræ, the sternum, and the ribs, and the yellow, which is found principally in the medullary cavities of the extremities. Small prolongations also extend into the larger Haversian canals.

The red marrow contains a rather delicate reticulum or network of connective tissue, embedded in the interstices of which are very numerous cells. These cells differ widely, and may be divided into five principal classes-viz., myelocytes, erythroblasts, eosinophiles, basophiles, and giant cells.

The myelocyte, myeloplax, or marrow cell, is a

moderate-sized cell capable of ameboid motion; it has a well-marked cell-body, and usually a single round nucleus. The cell-body frequently contains granules, which stain in neutral dyes (neutrophile granules). These cells are the most numerous in red marrow.

The erythroblasts, or hematoblasts, are rather smaller cells than the preceding, and contain a relatively

DEVELOPMENT OF BONE.

31

small cell body, yellowish in color, containing hemoglobin (the oxygen-carrying element of the red bloodcorpuscles) and a nucleus, which is frequently seen in process of division. These cells develop the colored corpuscles of the blood.

The eosinophiles, or acidophiles, resemble the myelocytes, but frequently have very irregularly shaped nuclei, and contain many large granules, which stain intensely with acid dyes (eosinophilic granules).

The basophilic cells, plasma cells, or mastzellen, resemble the eosinophiles excepting that the contained granules stain only in a basic dye. These cells occur only infrequently.

The giant cells are very large, irregularly shaped, ameboid cells, and usually contain many nuclei. They play an important part in the development of bone, and are then called osteoclasts. (See Developing Bone.) The nuclei of all the preceding varieties of cells are frequently seen in process of karyokinesis.

The yellow marrow differs from the red in that almost all the marrow-cells have been transformed into fat and the connective-tissue reticulum is slightly increased.

The marrow is well supplied with blood-vessels, lymphatics, and nerves, which come from the periosteum through the Volkmann's and the Haversian canals.

DEVELOPMENT OF BONE,

Bone may be developed in one of two ways: 1. From cartilage—the intracartilaginous development.

2. Under or between layers of periosteum-the intramembranous development.

In the embryo, all the bones of the body, with the

exception of a few in the head, are outlined by solid hyaline cartilage, and covered by a membrane-the primary periosteum-having the same structure as the perichondrium in other places. The development of adult bone from these cartilages begins at certain con

Fig. 11.-Developing bone, intracartilaginous: a, Periosteum; b, hyaline cartilage; c, small blood-vessel; d, osteoblast; e, osteoclast.

stant and well-defined points, which are called centers of ossification, and are usually situated a short distance from the ends of the bones. The cartilage-cells at these points enlarge considerably and arrange themselves in longitudinal rows, the matrix meanwhile increasing