depth in a straight or crooked direction through the leaves of a book, in which case it is plain that the perforations of the adjoining leaves would correspond; it being understood, however, that the passages thus formed are most likely bounded by proper parietes. The apertures now referred to must be distinguished from larger holes seen in some lamellæ, which give passage to the perforating fibres to be mentioned further on.

But the lamellæ have a further structure. To see this, the thinnest part of a detached shred or film must be examined, as shown in figs. 300 and 302; it will then appear plainly that they are largely made up of transparent fibres, decussating with each other in the form of an exceedingly fine network. In the Haversian systems these decussating fibres cross one another in different lamellæ at right angles (v. Ebner), but in most other situations at more or less acute angles, and they are united here and there by obliquely passing fibres, so that they cannot be teased out from one another; but at the torn edge of the lamella they may often be seen separate for a little way, standing out like the threads of a fringe. Most generally they are straight, as represented in fig. 300; but they are not always so; for in some parts they assume a curvilinear direction (fig. 302). Acetic or hydro

Fig. 300.-THIN LAYER PEELED OFF FROM A SOFTENED BONE, AS IT APPEARS UNDER A MAGNIFYING POWER OF 400 DIAMETERS (Sharpey).

This figure, which is intended to represent the reticular structure of a lamella, gives a better idea of the object when held rather farther off than usual from the eye.

Fig. 301.-SMALL PART OF A LONGITUDINAL SECTION OF DECALCIFIED TIBIA. HIGHLY MAGNIFIED (after v. Ebner).

a, series of six lamella which are cut for the most part in the direction of the fibrils, so that they appear longitudinally striated; b, b, lamellæ, the fibrils of which are cut across; the arrangement of the fibrils into bundles is indicated. Two lacunæ are seen lying between the lamella, also canaliculi piercing the lamellæ.

chloric acid causes these fibres to swell up and become indistinct, like the white fibres of connective tissue; care must therefore be taken in their examination that the remains of the decalcifying acid be removed from the tissue, by maceration in water or in solutions of neutral salts. Moreover, the fibro-reticular structure is not equally distinct in all parts; for in some places it is less decidedly marked, as if the fibrillation were incompletely developed.

The decussating fibres which constitute the lamellæ were discovered by Sharpey, and their constant presence was taught by him for a long time before they were admitted by other histologists. It has lately been shown by v. Ebner that the decussating fibres of Sharpey are in reality themselves composed of exquisitely fine fibrils, so that they correspond with bundles of white connective tissue fibres rather than with single fibres. Like the connective tissue fibrils these of the bone are doubly refracting, and they are said (Ebner) not to be calcified,

the deposit of calcareous matter being confined to the matrix in which they are embedded. They appear to be united into the lamellæ by a matrix or ground-substance, and take different directions in successive lamellæ, so as to produce a granular or a striated appearance according as they happen to be cut transversely or longitudinally (fig. 301).

In thin sections of bone, the concentric lines or rather bands which represent the cut edges of the lamellæ show the section of the decussating fibres as round or angular dots, themselves punctated, which lie embedded in the homogeneous ground-substance (fig. 301, b). The lamellæ are separated from one another by the lacunæ which lie between them, where these are absent they are joined together by the ground-substance; they are also united by occasional bundles of fibres passing obliquely from one lamella to the other.

Perforating fibres.-It was further shown by Sharpey that in many instances the lamellæ are perforated by fibres, which pass through them in a perpendicular or oblique direction, and, as it were, bolt them together. These perforating fibres may be seen, with the aid of the microscope, in a thin transverse slice of a decalcified cylindrical or cranial bone, on pulling asunder the sections of the lamellæ (as in fig. 304). In this way some lamella will generally be observed with fibrous pro

Fig. 302.-LAMELLE TORN OFF FROM A DECALCIFIED HUMAN PARIETAL BONE AT SOME DEPTH FROM THE SURFACE (Sharpey).

a, lamellæ, showing decussating fibres; b, b, thicker part, where several lamellæ are superposed; c, c, perforating fibres. Apertures through which perforating fibres had passed, are seen especially in the lower part, a, a, of the figure. Magnitude as seen under a power of 200, but not drawn to a scale (from a drawing by Allen Thomson).

cesses attached to them (fig. 304, b) of various lengths, and usually tapering and pointed at their free extremities, but sometimes truncated-probably from having been divided by the knife. These fibres have obviously been drawn out from the adjacent lamellæ, through several of which they must have penetrated. Sometimes, indeed, indications of perforations may be recognized in the part of the section of bone from which the fibres have been pulled out (fig. 304, c). The processes in question are thus, so to speak, viewed in profile; but they may frequently also be seen on the flat surface of detached lamellæ (fig. 302), projecting like nails driven perpendicularly.

The perforating fibres are, like the decussating fibres, for the most part bundles of fibrils which agree in character with those of the white fibrous tissue; but some, as shown by H. Müller, are of the nature of elastic tissue (fig. 303, e). In some parts they escape calcification, and thus, as they shrink in drying, leave tubes or channels in the dry bone, generally leading from the surface inwardly; but these

uncalcified fibres are by no means frequent (Sharpey). The perforating fibres are often connected with the periosteum, as is the case with most of those which penetrate the external table of the cranial bones; but in cross sections of cylindrical bones they often appear to spring with their broad ends from the deeper lamellæ (with the fibres of which they may be directly continuous), and especially from those near the circumference of a Haversian system, and taper outwards into fine points, which do not reach the periosteum (fig. 303), although without doubt they must, like the bony layers in which they occur, have been formed by subperiosteal ossification. They are never found in the concentric systems of Haversian

Fig. 303.-TRANSVERSE SECTION OF DECALCIFIED HUMAN TIBIA, FROM NEAR THE SURFACE OF THE SHAFT (E. A. S.).

H, H, Haversian canals, with their systems of concentric lamella; in all the rest of the figure the lamellæ are circumferential.

s, ordinary perforating fibres of Sharpey; e, e, elastic perforating fibres. Drawn under a power of about 150 diameters.

lamellæ. Perforating fibres exist abundantly in the crusta petrosa of the teeth (Sharpey).

Where tendons or ligaments are inserted into bone, the fibre-bundles of the tendon are continued into the bone as perforating fibres, so that the attachment of tendon to bone is thus rendered very intimate. Some of the bundles of white fibres of the periosteum may also, as above mentioned, pass into the bone as perforating fibres, and the same is the case with the elastic fibres.

The animal basis of bone is made up essentially, as we have seen, of lamellæ composed of fine decussating or reticular fibril-bundles embedded in a groundsubstance; but interposed among these lamellæ, layers are here and there met with of a different character, having a granular aspect, with the lacunæ very conspicuous and regularly arranged, and sometimes appearing as if surrounded by faintly defined areolæ. These generally incomplete layers are often bounded by a scalloped border, as if made up of confluent round or oval bodies; this is indicated also by the occa

sional occurrence of oval or flattened spheroidal bodies singly or in small groups near the border of the layers, each with a lacuna in the centre (fig. 305). In some parts the granular substance is obscurely fibrous, and transitions may be observed to the

Fig. 304.-MAGNIFIED VIEW OF A PERPENDICULAR SECTION THROUGH THE EXTERNAL TABLE OF A HUMAN PARIETAL BONE, DECALCIFIED (H. Müller).

At a, perforating fibres in their natural situation; at b, others drawn out by separation of the lamella; at c, the holes or sockets out of which they have been drawn (H. Müller).

well-marked reticular laminæ. The layers described appear principally to occur near the surface of the compact tissue, and at the circumference of many of the systems of concentric Haversian lamellæ.

Irregular layers of rounded bodies, apparently solid and without central cavity, are also sometimes seen, and are well represented in figure 306. These layers are

Fig. 305.-SMALL PART OF A SECTION

THROUGH THE SHAFT OF A FEMUR (HUMAN, 16 YEARS) TAKEN A SHORT DISTANCE FROM THE EPIPHYSIS. 230 DIAMETERS. (Kölliker.)

a, remains of calcified cartilage; b, bony deposit in Howship's foveola (absorption spaces); c, subsequent deposit of lamellar bone.

met with chiefly near the surface of the shaft of long bones, lying among the circumferential lamina, and apparently forming only part of a circuit. They can occasionally be recognised in a transverse section as short curvilinear bands of peculiar aspect, broader in the middle and thinning away at the ends, appearing here and there between the cut edges of two ordinary circumferential laminæ.

Finally, spaces are occasionally seen in a section of bone, which are characterized by an eroded outline, but in some cases they may be partially filled up by concentric

lamellæ. These were named "Haversian spaces" by Tomes and de Morgan, and they are interpolated or intruded amongst the regular Haversian systems, some of which may have been cut in upon in the excavation of the space. It was further noticed by Tomes and de Morgan that the spaces in question may sometimes be seen being filled up at one part by the deposition of lamellæ, whilst they are extending themselves by absorption at another. The Haversian spaces are most numerous in young and growing bones, but they occur also after growth is completed.

The three appearances above mentioned are due to the peculiar manner in which the absorption of bone occurs; for it is effected, as will presently be described, by

Fig. 306.-PORTION OF A NODULATED LAYER OF BONE-TISSUE FROM NEAR THE SURFACE OF THE SHAFT OF A DECALCIFIED HUMERUS (Sharpey).

At one side shreds of fibrous lamellæ are seen in the figure. Magnified 300 diameters. From a drawing by Allen Thomson.

the agency of large multinucleated cells, which excavate little hemispherical pits (foveola of Howship) in the osseous tissue. If the process of absorption should cease and should be succeeded by a re-deposition of osseous substance, the new osseous matter filling up the hollows of the absorbed surface exhibits, when it is detached, a raised impression corresponding with the hollows into which it fitted.

In young bones the lamellar character is far less distinct than in adult bones, the tissue being constituted chiefly of bundles of fibres which interlace in every direction in the ground-substance; in this reticular form of osseous tissue the lacunæ are both more numerous and irregular than in lamellated bone.

When tendons become ossified, as is often the case, especially in birds, little but e calcification of the ground-substance of the tendon occurs, so that, after decalcifying, the tendinous structure again becomes manifest.