sions and the groupings all tend strongly to the horizontal; the cells themselves are flattened in the same direction, till at last they become mere scales, three or four layers of which (fig. 5), lying close together, form the extreme free edge of the section; that is, the unattached surface of the articular cartilage consists of three or four layers of flattened cells, lying quite close together and overlapping each other's edges. Now, to suppose for one moment that the nutriment, which supplies growth to this body should be derived from the free surface, thus consisting of flattened cells, is to think directly contrary to all that we know of nutrition by cells. And therefore, as the growth of these cells evidently takes place from the attached portions, so is it evident that there must be some means of supplying nutriment to that surface. Moreover, the presence of large loops of vessels in the cancelli, immediately next the joint, points to the same fact.

Thus, it being clear that the nutriment of cells growing from a deep surface could not be derived from a superficial one, it becomes necessary to examine into the anatomy of the cartilage and bone, and their mode of attachment to each other. A paper, on the "Organization and Nutrition of Non-vascular Animal Tissues," by Mr. Toynbee, is published in the Philosophical Transactions' for 1841. The author describes the epiphysal cartilage, its vascularization and conversion into bone, and continues thus:

"In adult life, when the epiphysal cartilage has been ossified, the cancelli of the latter are separated from the articular cartilage by a layer of bone, to which may be given the name of articular lamella. The nature of this lamella is worthy of particular attention. It is composed of two sets of osseous layers, the one, dense and thick, is continuous with the vertical fibres of the cancelli; the other, delicate and thin, principally composed of osseous corpuscles, is situated at right angles to the latter, and fills up the interspaces of the vertical fibres. Is this articular lamella complete? I have never been able, by the aid of the microscope, to discover any orifices in it, nor have I been able to force mercury through it."

Todd and Bowman, in their Physiological Anatomy,' vol. i. p. 93, give Toynbee's account of this articular lamella, and refer to his paper. Even Kölliker, whose care and accuracy are so well known, has given an inexact account of this bony layer. He says:

66

"The condition of the bone, immediately under the articular cartilage, deserves especial mention; it consists, indeed, at almost every joint, of a layer of imperfectly-formed bone-matter, true bone-tissue being only found deeper. This layer, which is from 0:04 to 016 of a line in thickness, is formed of a yellow, generally fibrous substance of bony hardness, which, indeed, is really ossific, but which contains no trace of Haversian canals, and no formed lacunæ. Instead of the latter, one finds round or oval bodies lying together in heaps or rows, which in section appear very dark, and which therefore might be taken for bone corpuscles filled with granules of lime. By adding oil of turpentine this error may be avoided; and we find that, as with the real osseous lacunæ of dry bones, the dark appearance is due to air, and the formation in question consists of thick-walled granular cells, still retaining their contents (fat and granules), showing here and there traces of canaliculi and partly calcified; in other words, that they are undeveloped bone-cells."*

This is a circumstantial account, as far as the black bodies in the

• Mikroskopische Anatomie, Band i. p. 318.

articular lamella are concerned; they lie together in rows, between each member of which there is a layer, more or less thick, of the compact lamella, and thus there is no communication between each such cell (as there are no canaliculi), that is, fluid cannot pass through the dense bone material from one to the other, as Toynbee seems to have believed when he wrote that "this thin layer has already been stated to be almost entirely composed of osseous corpuscles, which, without doubt, assist to convey the fluid from the cancelli into the cartilage" (loc. cit. p. 172). Thus, according to the results to be obtained by studying the observations of the authorities on this subject, the bony cancelli are cut off entirely from any communication with the joint-cartilage by a dense ill-developed impenetrable structure.

In fact, although we have at one side of the articular lamella evident demand for nutriment, on the other evident arrangement for its supply, yet in the lamella itself there is no means of carrying the supply to the demand, not even the ordinary means which would have existed had it consisted of the usual bone-tissue with its lacunæ and canaliculi.

On making some pathological investigations on the diseases of cartilages, I could not avoid being profoundly impressed with the apparent contradiction above described, particularly as it threw some of the morbid processes under examination into even a worse confusion than itself laboured under. It thus became necessary, in my mind, that the nutrient mechanism of articular cartilage should be fully investigated, the present physiology and pathology of that substance being based upon notions, which, as they involve contradiction, must necessarily be either false or imperfect.

It first appeared desirable to make out the mode in which the cancellous structure close to the joint end of the bone was arranged, the more so as in the above mentioned Philosophical Transactions' the structure is figured as a set of regularly disposed dark squares, surrounded by lighter margins, like a series of picture-frames hung close together; and as room for tortuous vessels in such structure could hardly be imagined, I undertook a series of investigations to establish how their spongy texture was formed, how the vessels ran in it, and in what exact manner the lamella shuts out these cavities from the joint. For these purposes many sections were made of the joint ends of bone, and examined under a low magnifying power; it was then seen that the cancelli are not regular square cavities symmetrically arranged, but form large holes in the sections, of various shapes, and without any regularity of arrangement. The bony scales which divide these cavities are crowded with the ordinary bone cells and their branches, which, under a glass magnifying only ten diameters, look like little dots; the articular lamella has rather a lighter appearance, and even with so low a power may be seen to be of variable thickness. Fig. 1 represents a section of a part of the lower end of the human tibia. Several bones were examined belonging to different mammalia, rabbit, sheep, pig, ox, dog, horse, &c.; the only difference between them appears to be that the smaller the bone, the more compact is the tissue, more especially in little animals, as the rabbit, where the blood

channels are relatively not nearly so large and numerous as in the analogous joints of the human subject. (Fig. 1.)

A like section examined under a ten times higher power, shows that, besides the irregular large cancellous openings, there are running through the bony plates normal Haversian canals, with 10 diameters. their concentric system of laminæ

Fig. 1. Cancelli and articular lamella from

lower end of human tibia, magnified about

and lacunæ. The articular lamella is lighter in colour-i.e., more transparent than the rest of the bone; in it bone cells and canaliculi are absent, but there are several black opaque spots of an oblong form, with the long axis at right angles to the lamella, and two or three of these arrange themselves at a certain distance from each other in interrupted rows having the same direction. The lamella is a little darker, a little more brown, like ordinary bone, near its attached than its free surface; it looks as though at this part it had been stained. In no instance does the lamella lie immediately over, and never shuts in, a cancellous cavity; on the contrary, ordinary bone structure always intervenes between such cavity and the lamella. In places the osseous tissue surrounding a cavity near the margin of the lamella encroaches thereon very much; in other places, where the cavity is deeper from the surface the osseous tissue recedes, and the lamella projects into the bony structure; thus, the articular layer is very uneven in thickness, its free edge is also serrated rather finely but unevenly. (Fig. 2.)

In examining an injected preparation of the articular end of a bone it will be found that near the joint, immediately under the lamella, is a rich plexus of vessels, which forms a series of loops, and that in the curved portion of each loop the vessel appears dilated. This plexus is not in the long bones derived from the common nutrient arteries, but springs from some of the numerous branches, which surround the joint, and they are probably the remains of those vessels which Mr. Toynbee has described as supplying the epiphysal cartilage at the time of its ossification. In each one of the cavities near the articular lamella is a vascular twig, which does not entirely fill the space, but is surrounded more or less by a loose fibrous tissue and by fat. In no place does a vessel touch the articular lamella, ordinary bone-structure always intervening, as has been already stated.

On applying higher powers, one sees in many sections little more than this, particularly if they be mounted in Canada balsam; but in those, whether made by grinding down the bone or by cutting thin slices with a sharp knife, that are preserved in fluid, indica

tions were seen which tempted me to go on examining the structure of this articular lamella, until at last I convinced myself that it in reality consists of a series of very minute parallel tubes, which run in a wavy course from the bony to the cartilaginous surface. Among these, but having no special, if any, communication with them, are the

bodies mentioned by Kölliker as undeveloped bone cells. In some sectionsthose, namely, which are not made quite parallel with the axis of the joint from which they are taken-the tubes of the articular lamella cannot be made out, but the section is minutely dotted from those tubes having been cut across. Fig. 3 is a tolerably successful representation of this structure. It will be seen that certain portions of the articular lamella are rendered darker than others, and this is a condition very difficult to make out; yet perhaps I may pretty confidently affirm that it arises from the tubular structure having become so bent in those lines that the canals have been cut through, giving a brown, darker, and finely mottled character to that part. This structure is similar in every mammal in which I have examined the lamella, but perhaps it is plainer in the rabbit than in any other I have yet seen.

Fig. 3. Cancellous structure and articular lamella from upper end of the human humerus, showing tubular structure of the lamella, magnified 500

diameters.

Having thus succeeded in ascertaining the structure, as seen laterally, it seemed advisable to view the same part from above; for this

purpose a joint end, with as flat a surface as possible, was chosen; either end of the tibia in most animals answers this purpose sufficiently the cartilage being scraped away, a small piece of the articular lamella was detached, and ground thin enough to be transparent. In this view the black spots or undeveloped bonecells are less elongated, all the rest of the section is studded with dots, which, under a sufficient power, and where the section is very thin, appear as small round holes. This structure is the same in all animals that I have examined. (Fig. 4.)

Thus the articular lamella, supposed until now to be a compact, impenetrable layer of bone, is in reality a structure as tubular as dentine, but the tubes are much finer and less straight; they do not shine as black as the lines in dentine, probably because they are finer, and therefore do not refract light to the same degree. They are, I believe, the minutest set of tubes yet discovered in the body, certainly in any of the hard portions, and some of the tubes lately reported to exist in the softer parts are, to say the least, doubtful. It is necessary that they should be sought for under a high power and with a very good light, which can be varied by Gillott's condenser, or by other

means.

Articular cartilage has been so often described, that it must be needless to do more than refer to the sketch already given of the method in which the cells

lie, and again to insist upon the fact of the greater crowding of these bodies near the superficial surface, and on their ultimate drying into scales, which overlie each other on that surface-an arrangement which has caused many observers to believe in the existence of an epithelium. If the superficies of fresh cartilage be shaved off thin with a very sharp knife, the section will indeed have the appearance of a layer of epithelial cells; but if a thin slice through its substance be examined, the gradual horizontal arrangement and flattening of the cells will leave no doubt as to the true structure of its superficies. (Fig. 5.)*

Fig. 5. Cartilage from human astragalus, magnified about 700 diameters, showing the perpendicular arrangement of corpuscles at the lower part, gradually curving into an oblique, subsequently into a horizontal, position, and drying into scales.

Some observers, Mr. Toynbee among them, found that in the foetus vessels run across the cartilage, even into the middle of joints. In neither a fœtal hare nor calf that I had the opportunity of examining, could I discover any such arrangement, nor any trace of it in a still

This arrangement has, however, not been described as is here done, because it is so difficult to procure sections sufficiently thin, that go all through the cartilage, the outer layers of cells breaking easily from a very thin slice. It may best be managed by cutting away cartilage and articular lamella from the cancelli, scraping the osseous matter away, then laying the cartilage on a piece of cork, and beginning from the formerly attached surface, cutting slices with a well made and sharp Valentin's knife.