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fibres pari passu with their growth, and thus new bony spicules become continually formed by calcification of the groups or bundles of osteogenic fibres.

The earthy deposit occasionally appears in an isolated patch here and there on some of the osteogenic fibres in advance of the main area of ossification (see fig. 309, A, a).

The osteogenic fibres become comparatively indistinct as they and the substance between them calcifies; they appear, however, to persist in the form of fine fibres, such as are seen in the lamellæ of the adult bone, although in the embryonic bone their disposition is not lamellated, the bony matter having a somewhat coarsely reticular structure.

In this way the first bony matter becomes formed as a perforated plate or network of osseous spicules, which, whilst becoming extended peripherally in the way above described, gradu

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Fig. 309.-PART OF THE DEVELOPING PARIETAL BONE OF A FETAL CAT (1 INCH LONG). (From drawings by Mr. J. Lawrence.) (E. A. S.)

A, a piece of the growing edge slightly magnified, showing the bony spicules terminated by bunches of osteogenic fibres; a, an isolated bony spicule united to the main part of the ossification by a bundle of osteogenic fibres.

B, the part marked b of the smaller figure, highly magnified; sp, bony spicules, with some of the osteoblasts imbedded in them, producing the lacunæ; ost, osteoblasts partly imbedded in the newly formed bone; of, osteogenic fibres prolonging the spicules, with osteoblasts between them and applied to them; a, granules of calcareous deposit between the osteogenic fibres; at b the granules have become blended, and the matrix is clearer; at c a continuity is established between the two adjacent spicules.

fibres. The perforations in these first-formed bony plates correspond to the bays which were seen between the advancing spicules, and to the meshes of the bony network formed afterwards by the junction of the spicules, and as the bone thickens they become enclosed and converted into reticulating interstices (like the canals of a sponge), which are occupied by blood-vessels, and by the corpuscles above mentioned. These corpuscles also everywhere cover the osteogenic fibres, to which their flattened sides are often applied (fig. 309, B, ost). Where the osteogenic fibres diverge from one another, the intervals are occupied by the same cells. It is probable that the osteogenic substance is formed by the agency of the cells in question, hence the name "osteoblasts" was assigned to them by Gegenbaur.

Some of the osteoblasts are involved in the ossifying matrix, and remain as the corpuscles of the future bone, the spaces enclosing them being the lacunæ. It is supposed that the canaliculi, which are at first short, are afterwards extended by absorption, so as to anastomose with those of neighbouring lacunæ.

It is believed by many histologists that the fibrillated ground-substance of bone is formed not outside the cells in an intercellular substance, but by a direct conversion of the protoplasm of some of the osteoblasts into bony tissue. If this were the case, there ought to be some indication in the formed osseous substance of the cell-areas of which it was made up, but nothing of the kind has been shown to exist. There should moreover often be observed osteoblasts which are only partly converted into bony substance, but this has also never been described. And if as some suppose, the peripheral part of each osteoblast becomes converted into osseous substance, while the central part and nucleus remain as the corpuscle within a lacuna, the osteoblasts would have to be originally far larger than the permanent lacunæ, which is certainly not the case. The view in question is similar to that which supposes ordinary connective tissue to have a like origin, and appears to rest more upon theory than on actual observation of the stages of the developmental process.

Meanwhile, the meshes of the bony network, which were occupied as we have seen by one or more blood-vessels, and by numerous osteoblasts, become diminished in extent, and the bone at the same time increased in thickness by the deposit upon the original trabeculæ of irregular bony laminæ and trabeculæ, some of the osteoblasts remaining, and forming the corpuscles and lacunæ as before. The interstices of the bony spongework thus becomes gradually narrowed, containing one or more blood-vessels surrounded by osteoblasts.

At a later stage the increase in thickness takes place by successive depositions of bony lamellæ under the periosteum, a concentric deposition occurring at the same time on the walls of the vascular channels. But since the growth in thickness of a membrane-bone takes place in exactly the same manner as that of one of the long bones, which will be fully described in a subsequent page, the reader is referred to the account of the process there given.

It may be observed that the appearance of the ossifying membrane-bone in the shape of a network of trabeculæ seems to be determined by the pre-existence of a vascular network in the embryonic tissue. The new bone everywhere makes its appearance in the spots which are furthest from the vessels, and the bony network everywhere alternates with the vascular network. At the edges of the advancing bone the spicules which prolong it pass between, and avoid the capillary bloodvessels, which are thus left in the bays between the spicules : the divergent bunches of osteogenic fibres which prolong the adjacent spicules complete the enclosure of the blood-vessel.

After a time the membrane-bone extends so as almost to come into contact with the neighbouring bones. But as long as growth continues, there always remains in the situation occupied afterwards by the sutures a vascular connective tissue with numerous osteoblasts. This is continually on the increase, but as fast as it grows, the osteogenic fibres and the osseous spicules extend into it from the young bones on either side. At length, however, when these have attained their full dimensions, the growth of the intermediate tissue ceases, and it becomes completely invaded by the bone on either side, with the exception of the narrow and irregular line of suture, which may eventually itself become more or less obliterated.

From a morphological point of view, the membrane-bones, especially those of the skull, are probably to be regarded as the modified remains of an integumental skeleton which is extensively developed in some of the lower vertebrata, and which had in all probability as its phylogenetic precursor a formation of dentinous, cutaneous spines. Even the membraneformation in connection with the cartilage-bones may have originated in the same manner.

OSSIFICATION IN CARTILAGE.

It has already been stated that, in by far the greater number of bones the mesodermic tissue with closely packed cells, of which they originally consist, is very quickly succeeded by cartilage, in which the ossification begins. One of the long bones taken from a very small embryo, just before ossification has commenced

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Fig. 311.-SECTION OF PHALANGEAL BONE OF HUMAN FETUS, AT THE TIME OF COMMENCING OSSIFICATION. Photographed from a preparation by Mr. F. A. Dixey. Magnified about 75 diameters. (E. A. S.)

The cartilage cells in the centre are enlarged and separated from one another by dark-looking calcified matrix; im, layer of bone deposited underneath the periosteum; o, layer of osteoblasts by which this layer has been formed. Some of the osteoblasts are already embedded in the new bone as lacunæ. The cartilage-cells are becoming enlarged and flattened and arranged in rows above and below the calcified centre. At the ends of the cartilage the cells are small and the groups are irregularly arranged; the fibrous periosteum is not sharply marked off from the cartilage.

in it, is observed to be distinctly cartilaginous. In the tibia of a sheep, for example, at a time when the whole embryo is not more than an inch and a quarter in length, we can plainly see that the substance consists of cartilage-cells imbedded in a pellucid matrix. These cells can scarcely be said to be collected into groups, and are very irregular in size and shape. They become enlarged in the middle part of

the shaft when ossification is about to commence. As it grows, the cartilage acquires firmer consistence; it represents in figure the future bone, though of course much smaller in size, and it is surrounded with a fibrous membrane, the future periosteum. Vessels ramify in this membrane, but none are seen in the cartilage until ossification is about to begin. In a long bone the ossification

Fig. 312.-SECTION OF PART OF ONE OF THE

LIMB BONES OF A FETAL CAT, AT A
MORE ADVANCED STAGE OF OSSIFICATION
THAN IS REPRESENTED IN FIG. 311, AND
SOMEWHAT MORE HIGHLY MAGNIFIED.
(Drawn by Mr. J. Lawrence.) (E.A.S.)

The calcification of the cartilage matrix has advanced from the centre, and is extending between the groups of cartilage-cells which are now arranged in characteristic rows. The subperiosteal bony deposit (im) has extended pari passu with the calcification of the cartilage matrix. The cartilage cells in the primary areolae are mostly shrunken and stellate, in some cases they have dropped out of the space. At ir and in two other places an irruption of the subperiosteal tissue, composed of ramified cells with osteoblasts and growing blood-vessels, has penetrated the subperiosteal bony crust, and has begun to excavate the secondary areolae or medullary spaces; p, fibrous layer of the periosteum; o, layer of osteoblasts, some of them are embedded in the osseous layer as bone-corpuscles in lacunæ ; bl, bloodvessels occupied by blood-corpuscles. Beyond the line of ossific advance the periostum may be noticed to be distinctly incurved. This incurvation is gradually moved on, the cartilage expanding behind it until the head of the bone is reached, when it forms the periosteal notch or groove represented in fig. 313, p.

[graphic]

commences in the middle and proceeds towards the ends, which remain long cartilaginous, as represented in fig. 310. Much later, separate points of ossification appear in them, and form epiphyses, which at last are joined to the body of the bone.

bl

im

The manner in which the process of ossification of a cartilage bone takes place. is as follows:

In the middle of the cartilage the cells are enlarged, and are separated from one another by a relatively larger amount of matrix than elsewhere (fig. 311). This matrix becomes hardened by calcareous deposit, assumes a granular opaque appearance, and has a gritty feel to the knife. Meanwhile the cartilage-cells above and below the centre of ossification become enlarged and flattened, and piled up in elongated groups or columns which radiate from the centre for a certain distance towards either end. The columns taper towards their ends, where the cartilage-cells which compose them are smaller. Into the matrix between these oblong groups the calcareous deposit extends between and around the groups of cells, so that the calcified substance encloses the columns; the cell-spaces in the calcified matrix

which are still occupied by the cartilage-cells, either singly or in elongated groups, being termed the primary areola (Sharpey). Simultaneously with this deposit in the cartilage-matrix, a layer of osseous substance (fig. 311, im) is becoming formed on the outside of the cartilage underneath the periosteum. This last is a vascular membrane, containing numerous osteoblasts (o), which are chiefly collected on the

bv

ic

im

Fig. 313.-LONGITUDINAL SECTION THROUGH

THE UPPER HALF OF THE DECALCIFIED
HUMERUS OF A FETAL SHEEP, AS SEEN
UNDER A MAGNIFYING POWER OF ABOUT

30 DIAMETERS. (Drawn by Mr. J.
Lawrence.) (E.A.S.)

ic, the part of the shaft which was primarily ossified in cartilage; what remains of the primary bone is represented as dark, enveloped by the clear secondary deposit. The areola of the bone are occupied by embryonic marrow with osteoblasts, and blood-vessels variously cut, represented as dark lines. One long straight vessel (be) passes in advance of the line of ossification far into the cartilaginous head, most of the others loop round close to the cartilage. At one or two places in the older parts of the bone elongated groups of cartilage-cells (c) may still be seen which have as yet escaped absorption. im, the part of the bone that has been ossified in membrane, that is to say in the osteoblastic tissue under the periosteum. It is well marked off from the central portion, and is bounded, peripherally, by a jagged edge, the projections of which are indistinctly seen to be prolonged by bunches of osteogenic fibres. A row of osteoblasts covers the superficial layer of the bone. The subperiosteal layer is prolonged above into the thickening (p), which encroaches upon the cartilage of the head of the bone, and in which are seen, amongst numerous osteoblasts and a few blood-vessels, the straight, longitudinal osteogenic fibres (of), and some other fibres (pf) crossing them, and perhaps representing fibres of Sharpey. The calcareous salts having been removed by an acid, the granular ossific deposit passing up between the rows of cartilage-cells is not seen in this specimen. Observe the general tendency of the osseous trabeculæ and the vascular channels between them to radiate from the original centre of ossification. This is found to prevail more or less in all bones when they are first formed, although the direction of the trabeculæ may afterwards become modified in relation with varying physiological conditions, and especially as the result of pressure in different directions.

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inner surface next to the cartilage, and it is by their agency that the bony layer on the surface of the cartilage is formed and becomes increased both in thickness and length. The bony layer, when viewed on the surface, shows the usual component fibres of bony lamellæ, and as other layers are deposited upon it lacunæ become formed between them by the inclusion of some of the osteoblasts. In this first stage of ossification, we see therefore two processes going on, a deposit of earthy

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