or less degree, and that the difference between the two varieties of tissue depends on the different amount of solid matter compared with the size and number of the open spaces in each; the cavities being very small in the compact parts of the bone, with much dense matter between them; whilst in the cancellated texture the spaces are large, and the intervening bony partitions thin and slender. There is, accordingly, no abrupt limit between the two,-they pass into one another by degrees, the cavities of the compact tissue widening out, and the reticulations of the cancellated

Fig. 295.-LONGITUDINAL SECTION THROUGH THE UPPER END OF THE FEMUR SHOWING THE CANCELLOUS STRUCTURE OF THE HEAD AND THE COMPACT SUBSTANCE OF THE SHAFT. (From a photograph by Zaaijer.)

becoming closer as they approach the parts where the transition between the two takes place.

In all bones, the part next the surface consists of compact substance, which forms an outer shell or crust, whilst the spongy texture is contained within. In a long bone, the large round ends are made up of spongy tissue, with only a thin coating of compact substance (fig. 295); in the hollow shaft, on the other hand, the spongy texture is scanty, and the sides are chiefly formed of compact bone, which increases in thickness from the extremities towards the middle, at which point the girth of the bone is least, and the strain on it greatest. In tabular bones, such as those of the skull, the compact tissue forms two plates, or tables, as they are called, inclosing between them the spongy texture, which in such bones is usually named diploë. The short bones, like the ends of the long, are spongy throughout, save at their

surface, where there is a thin crust of compact substance. In the complex or mixed bones, such as the vertebræ, the two substances have the same general relation to each other; but the relative amount of each in different parts, as well as their special arrangement in particular instances, is very various.

On close inspection the cancellated texture of bone is seen to be formed of slender bars or spicula and thin lamellæ, which meet together and join in a reticular manner, producing an open structure which has been compared to lattice-work (cancelli), and hence the name usually applied to it. In this way considerable strength is attained without undue weight, and it may usually be observed that the

Fig. 296.-A, TRANSVERSE SECTION OF A BONE (ULNA) DEPRIVED OF ITS EARTH BY ACID (Sharpey) The openings of the Haversian canals are seen. Natural size. A small portion is shaded to indicate the part inagnified in Fig. B.

B, PART OF THE SECTION A, MAGNIFIED 20 DIAMETERS.

The lines indicating the concentric lamella are seen, and among them the lacunæ appear as little dark specks.

strongest laminæ run through the structure in those directions in which the bone has naturally to sustain the greatest pressure. The open spaces or areolæ of the bony network communicate freely together; in the fresh state they contain marrow and blood-vessels.

Haversian canals.-The compact tissue is also full of holes; these, which are very small, are best seen by breaking across the shaft of a long bone near its middle and examining it with a common magnifying glass. Numerous little round apertures (fig. 296 A) may then be seen on the broken surface, which are the openings of short longitudinal passages running in the compact substance, and named the Haversian canals, after Clopton Havers, an English physician and writer of the seventeenth century, who more especially called attention to them. Blood-vessels run in these canals, and the widest of them also contain marrow. They are from 10th to th of an inch in diameter: there are some no more than 20th, but these are

rare; the medium size is about th. The widest are met with nearest the medullary cavity, and the narrower towards the circumference of the bone. They are quite short, as may be seen in a longitudinal section, oblique communications connecting them freely both longitudinally and laterally. Those which are next the circumference of the bone, open by minute pores on its external surface, and the innermost ones open widely into the medullary cavity; so that these short channels. collectively form a sort of irregular network of tubes running through the compact tissue, in which the vessels of that tissue are lodged, and through the medium of which these vessels communicate together, not only along the length of the bone, but from its surface to the interior through the thickness of the shaft. The canals of the compact tissue in the other classes of bones have the same general characters, and for the most part run parallel to the surface.

Fig. 297.-SECTION OF A HAVERSIAN CANAL, SHOWING ITS
CONTENTS. HIGHLY MAGNIFIED. (E. A. S.)

a, small arterial capillary vessel; v, large venous capillary; n, pale nerve-fibres cut across: 7, cleft-like lymphatic vessel: one of the cells forming its wall communicates by fine branches with the branches of a bone-corpuscle. The substance in which the vessels run is connective tissue with ramified cells; its finely granular appearance is probably due to the cross section of fine fibrils.

Most of the Haversian canals contain two small blood-vessels, arterial and venous (fig. 297), together with a small amount of delicate connective tissue containing branched cells, which are flattened close to the bone, and communicate by their branches with the ramifications of corpuscles in the substance of the bone.

Lamellæ.-On viewing a thin transverse section of a long bone with a microscope of moderate power, especially after the earthy part has been removed by acid (fig. 296 B; fig. 297), the opening of each Haversian canal appears to be surrounded by a series of concentric rings. This appearance is occasioned by the transverse sections of concentric lamella which surround the canals. The rings are not all complete, for here and there one may be seen ending between two others. In some of the sets, the rings are nearly circular, in others oval,-differences which seem mostly to depend on the direction in which the canal happens to be cut: the aperture too, may be in the centre or more or less to one side, and in the latter case the rings are usually narrower and closer together on the side towards which the aperture deviates. Again, some of the apertures are much lengthened or angular in shape, and the lamellæ surrounding them have a corresponding disposition. Besides the lamellæ surrounding the Haversian canals, there are others disposed conformably with the circumference of the bone (fig. 296 B, a); most of these are near the surface, but others run between the Haversian sets, by which they are interrupted in many places (fig. 303). Lastly, in various parts of the section, lines are seen which indicate lamellæ, differing in direction from both of the above-mentioned orders.

The appearance in a longitudinal section of the bone is in harmony with the account above given the sections of the lamellæ are seen as straight and parallel lines, running in the longitudinal direction of the bone, except when the section. happens to have passed directly or slantingly across a canal: for wherever this occurs there is seen, as in a transverse section, a series of rings, generally oval and much lengthened on account of the obliquity of the section.

Many of the Haversian canals which pass through the circumferential or periosteal lamellæ carrying blood-vessels from the periosteum into the bone, are not surrounded by concentric

lamellæ, but are mere channels piercing the periosteal lamellæ. They are often spoken of as Volkmann's canal.

The cancellated texture has essentially the same lamellar structure. The slender bony walls of its little cavities or areola are made up of superimposed lamellæ, like those of the Haversian canals, only they have fewer lamellæ in proportion to the width of the cavities which they surround; and, indeed, the relative amount of solid matter and open space constitutes, as already said, the only difference between the two forms of bony tissue: the intimate structure of the solid substance and the manner of its disposition round the cavities being essentially the same in both.

Lacunæ and canaliculi.—All over the section numerous little dark specks are seen among the lamellæ. These were named the "osseous corpuscles;" but as it is now known that they are in reality minute cavities existing in the bony substance, the name of lacunae has since been more fittingly applied to them. To see the lacunæ

Fig. 298.-TRANSVERSE SECTION OF COMPACT TISSUE (OF HUMERUS), MAGNIFIED ABOUT 150 DIAMETERS (Sharpey).

Three of the Haversian canals are seen, with their concentric rings; also the lacunæ, with the canaliculi extending from them across the direction of the lamellæ. The Haversian apertures had become filled with air and débris (from the grinding), and therefore appear black in the figure, which represents the object as viewed with transmitted light.

properly, however, sections of unsoftened bones must be prepared and ground very thin, and a magnifying power of from 200 to 300 must be employed. Such a section, viewed with transmitted light, has the appearance represented in fig. 298. The openings of the Haversian canals are seen with their encircling lamellæ, and among these the lacunæ, which are mostly ranged in a corresponding order, appear as black, opaque or nearly opaque oblong spots, with fine dark lines extending from them and causing them to look not unlike little black insects. The dark appearance is due to the fact that the little cavities have become filled with air in the dry bone, and when the same section is seen against a dark ground, with the light falling on it (as we usually view an opaque object), the little bodies and lines appear quite white, like figures drawn with chalk on a slate, and the intermediate substance, being transparent, now appears dark.

The lacunæ, as already stated, are minute recesses in the bone, and the lines extending from them are fine pores or tubes named canaliculi, which issue from their

cavity. The lacunæ present some variety of figure, but in such a section as that represented they for the most part appear irregularly fusiform, and lie nearly in the same direction as the lamellæ between which they are situated; or, to speak more correctly, they are flattened and extended conformably with the lamellæ ; for when the bone is cut longitudinally, their sections still appear fusiform and are still more lengthened out in the direction of the lamellæ. The canaliculi, on the other hand, pass across the lamellæ, and they communicate with those proceeding from the next range of lacunæ, so as to connect the little cavities with each other; and thus since the canaliculi of the most central range open into the Haversian canal, a system of continuous passages is established by these minute tubes and their lacunæ, along which fluids may be conducted from the Haversian canal through its series of surrounding lamellæ; indeed, it seems probable that a chief purpose of these minute passages is to allow nutrient matter to be conveyed from the vascular Haversian canals through the mass of hard bone which lies around and between them. In like manner the canaliculi open into the great medullary canal, and into the cavities of the cancellated texture; for in the thin bony parietes of these cavities lacunæ are also contained; they exist, indeed, in all parts of the bony tissue. The canaliculi which radiate outwards from the lacunæ near the periphery of the Haversian systems do not as a rule communicate with those of the neighbouring Haversian system, but bend round and are joined to one another.

Cells of bone.-As first shown by Virchow, each lacuna is occupied by a flattened nucleated cell, which sends branches along the canaliculi; and later observers (Rouget, Neuman,) have been able to detach the proper wall of the lacuna

Fig. 299.-A BONE-CELL ISOLATED AND HIGHLY MAGNIFIED

(after Joseph).

a, proper wall of the lacuna, shown at a part where the corpuscle has shrunk away from it.

and its appertaining canaliculi after decalcification, and to obtain it separate with its included corpuscle (fig. 299). It can scarcely be doubted that the protoplasm of the nucleated corpuscle takes an important share in the nutritive process in bone, and very probably serves both to modify the nutritive fluid supplied

from the blood and to further its distribution through the lacunar and canalicular system of the bony tissue. Virchow showed that the corpuscles of bone are homologous with those of ordinary connective tissue: to this it may be added that the enclosing lacunæ and canaliculi are to be looked upon as corresponding to the cellspaces of that tissue.

Apertures and decussating fibres of the lamella.-With a little pains thin films may be peeled off in a longitudinal direction from a piece of bone that has been softened in acid. These for the most part consist of several lamellæ, as may be seen at the edge, where the different layers are usually torn unequally, and some extend farther than others. Examined in this way, under the microscope, the lamellæ are seen to be perforated with fine apertures placed at very short distances apart. These apertures were described by Deutsch1; they appear to be the transverse sections of the canaliculi already described, and their relative distance and position accord sufficiently with this explanation. According to this view, therefore, the canaliculi might (in a certain sense) be conceived to result from the apposition of a series of perforated plates, the apertures of each plate corresponding to those of the plates contiguous with it; or they might be compared to holes bored to some

1 De Penitiori Ossium Structurâ. Wratisl. 1834, p. 17, Fig. 6.