chronic rheumatism, where the articular ends of bone lose their cartilage and become hard and polished like ivory, owing to the blocking up of the Haversian canals by bone.

LACUNA cha

bone.

The "lacuna" are the spider-like cavities which we racteristic of true find between the laminæ, arranged in concentric circles round the Haversian canals. They are characteristic of true bone, and distinguish it from accidental "ossifications," sometimes met with as products of disease. Formerly the lacunæ and canaliculi, in consequence of their dark colour, were considered to be solid; but subsequent observation has proved them to be hollow, since they can be filled with turpentine or Canada balsam. If, for example, a drop of oil of turpentine be applied to a section of dry bone under the microscope, capillary attraction will cause the fluid to enter the canaliculi, and its passage may be traced from one lacuna to another. It is a curious circumstance, that, in the bones of bodies that have been embalmed, the bone cells and canaliculi are filled with the bituminous material.

As a rule, the lacunæ are oval and flattened, so that one of their broad sides may be turned towards the Haversian canal. The first ring of lacunæ sends half of its canaliculi directly into the Haversian canal, while the other half communicate with the canaliculi of the second ring, and so on throughout the whole system. The nutrient fluid, or "plasma," of the blood, exuding through the coats of the blood-vessel in the Haversian canal, is imbibed by the nearest row of lacunæ, and passed on from them to all the others. in the Haversian system. One may say then that the lacunæ are

reservoirs of nutrition for the bone.

of an in

Their size and In man, the lacunæ measure about shape. inch in their long diameter, and about their short. It has been shown by Mr. Quekett that they vary in size and shape in the four great classes of animals, so that by means of this test it can be ascertained with certainty whether a given fragment of bone be part of a mammal, a bird, a reptile, or a fish. As this test is equally applicable in the case of fossil bones, it has an important bearing upon the study of geology. Another interesting fact discovered by Mr. Quekett is, that the size of the lacunæ bears very little relation to the size of the animal

to which they belong. They are nearly as large in the bones of the little lizard, as they are in those of the enormous extinct lizard, the Iguanodon. But their size does bear an exact proportion to that of the blood cells in the several classes of animals. Therefore, as reptiles have the largest blood cells, so have they also the largest lacunæ.

office.

CANALICULI: Respecting the "canaliculi" (Plate B. fig. 5), their size and observe how exceedingly minute they are; that they run off from all parts of the circumference of the lacunæ, and communicate most freely with the canaliculi of the adjoining lacunæ. Their diameter ranges from A of an inch to of an inch; but there are some even smaller. They are far too small to allow the entrance of blood cells. They admit the passage of nothing but a thin juice from the blood, the "plasma," destined to nourish the bone and keep it in a state fit for self-repair when injured by disease or violence.

LAMINÆ.

In man, and almost all mammalian animals, bone grows by the deposit of fresh layers. In all cases the new layer is deposited on that surface of the old layer which is next to the blood-vessel. Therefore, in a fully formed Haversian system, we get the appearance of "concentric" rings. They vary in thickness from 3000 to 5000 of an inch. Those around the Haversian canals vary from five to fifteen in number, and are called the "Haversian lamina." Those surrounding the circumference of a long bone which has reached its full growth, are termed "circumferential lamina" (Plate B, fig. 4 a). The illdefined and broken layers apparent here and there in the Haversian interspaces are termed "interstitial lamina" (Plate B, fig. 4 b). It seems doubtful how these interstitial laminæ were originally formed; but the recent investigations of Messrs. Tomes and De Morgan lead them to believe that they are the remnants of Haversian systems that have been partially removed by absorption. Structure of a If a well-marked lamina be examined with a lamina. power of 440 diameters, we shall find that it consists of two portions-an inner, apparently structureless and clear; and an outer, granular and opaque (Plate C, fig. 1). This difference of structure arises from the manner in which the several

ingredients of bone are originally laid down. Here we must bear in mind that the animal matter of the bone, in its nascent state, consists of a soft, fibrous tissue, which we call "intercellular," and of a number of cells called "osteal cells." The animal matter is laid down first; the earthy part, consisting of granules, is added afterwards; and both intercellular substance and cells become ossified. Now the cells, when ossified, present a much more granular and opaque appearance than the ossified intercellular substance. Therefore, if the cells, in place of being scattered here and there promiscuously, arrange themselves in a row close together, leaving the intercellular matrix clear, the new layer of bone will present a linear appearance, there will be a layer of opaque or granular bone, and a layer of more transparent bone, Lamination : To use the words of Messrs. Tomes and De Morgan*, what is it? "Lamination is but a definite linear arrangement of the osteal cells with their outlines permanently retained in the perfected bone."

OSSEOUS GRANULES.

The earthy salts are deposited in the animal matrix in the form of exceedingly minute granules. The Germans call them "bone crumbs." We cannot see them, however, without a magnifying power of 1200 diameters (Plate B, fig. 5). They vary in size in different specimens of bone. In man their size ranges from 0 to 14000 of an inch. They can be very distinctly seen in the skulls of small birds—the canary, for instance and also in the skull of the bat, where they are so much larger than in the human subject. After a section of bone has been steeped for some time in dilute hydrochloric acid, these earthy particles will be dissolved out of the animal matrix, and the little cavities in which they are imbedded can then be distinctly seen.

Present in pus It is an interesting and valuable practical fact, coming from dead that these earthy granules are generally present in the pus which comes from dead bone. If a specimen of pus under such circumstances be examined with a

bone.

* Philosophical Transactions, 1853.

C 3

power of 500 diameters, a number of earth granules may be detected among the pus cells, proving that there is dead bone somewhere. Mr. Quekett noticed this fact many years ago. Mr. Bransby Cooper has also ascertained that in pus coming from diseased bone there is as much as two and a half per cent. of phosphate of lime.

HAVERSIAN

SPACES.

This name has been given by Messrs. Tomes and De Morgan to certain spaces which they have lately discovered in bone (Plate C, fig. 2). The spaces are produced by absorption of old bone, and in process of time are again filled up with deposits of new laminated bone. This discovery is important, since it shows that bone is not a stationary structure; on the contrary, it is subject to a kind of absorption and reproduction of its tissue, throughout life, the activity of the process diminishing with advancing age. How are we to distinguish these "Haversian spaces" from "Haversian canals?" As follows:The walls of the Haversian canals have a smooth regular outline; on the other hand, the Haversian spaces have a festooned irregular boundary, apparently produced by the absorption of parts of several Haversian systems. Indeed, the authors just mentioned consider that the "interstitial lamina" are but the remains of Haversian systems nearly all absorbed.

How distinguished from Haversian canals.

"Haversian spaces" may be found in various conditions. In some the process of absorption is still going on; in others the deposit of new bone has commenced. Thus, then, it would seem that portions of the circumference of three or four Haversian systems, having lived a certain time, are gradually removed, and presently a new Haversian system is set up in their place.

ARTICULAR

liarities.

By this we understand a thin layer of bone BONE: Its pecu- situated immediately under articular cartilage; and since there is a peculiarity about the structure of it, we will allude to it here. If a perpendicular section be made through the articular surface of any fresh bone with the cartilage attached, it will be observed, (as seen in the cut,) that the cartilage does not rest immediately upon the cancellous tissue of the bone, but upon a thin compact crust of bone which closes the cancelli.

This crust, which we call "articular bone," varies in thickness, and is of a remarkably white colour. But its chief peculiarity consists in this, that it has

no Haversian canals in it, and and therefore is not vascular. The blood-vessels of the cancellous tissue run up only as high as its under surface, and then turn back in loops. Moreover, its "bone cells" are three or four times larger than in ordinary bone, and are destitute of canaliculi. Here is a striking instance of design, This layer of bone, having no

FIG. 7.

a Upper Cartilage cells.

b Lower Cartilage cells.

c Articular Bone.

d Bone of the Shaft.

Haversian canals, is much less porous than common bone, and in consequence of its closer texture is all the stronger, and more adapted to form an unyielding surface for the support of the articular cartilage.

Although articular bone and adult articular cartilage have no blood-vessels in health, yet they both become vascular in some cases of disease of the cartilage. Blood-vessels may be seen, when successfully injected, shooting up through the heretofore nonvascular layer of bone into the cartilage on its surface.

or in membrane.

Bone may be Remember what bone is: A matrix or bed of formed in cartilage animal matter, hardened by a deposit of earthy salts. The animal matter may be cartilage, or membrane. Hence we have formation of bone in cartilage, and formation of bone in membrane. These subjects must be considered separately; and in order to a right understanding of the first of the two, it is essential to know something of the nature of cartilage.