In a specimen of diarthrodial cartilage, taken from an adult mammal, if we make a thin section parallel to the articular surface, and look directly upon this part of the interior of the joint, we see appearances similar to those represented infig. 402. A number of cartilage corpuscles, at irregular distances from each other, and separated by the intercellular sub stance of this tissue, constitute the only cell-formation visible, and the existence of similar corpuscles at varying depths in the substance of the cartilage may easily be verified. The chief difference noticeable between the deeper and more superficial of these cells is, that those in the latter situation contain in their interior many yellow and highly refractile granules, which are of com paratively uniform size, and occupy their cavity about midway between their tolerably central nucleus and the inner surface of the cell-membrane. This appearance becomes still more manifest as the corpuscles approach the articular surface. A thin vertical section of the cartilage shows that the cells are in greater numbers near this surface, and the edge which borders the joint exhibits an irre gular outline, from which cells may often be seen projecting. The attrition which these appearances would seem to denote appears to be exerted upon the cells equally with the interstitial substance of the cartilage, but is rnore difficult to verify in the former tissue, since such a cell that has suffered a partial destruction of its form, has, at the same time, lost a valuable optical means of detection. Occasionally, h owever, as infig. 402., on looking directly at the free surface of the tissue, we see a darkish nucleus, lying very superficially, and surrounded by a clear space. In all pro bability, this was such a cell ground down to a hemispherical cavity. More rarely, a profile view of such a hemisphere is obtained.
On examining similar specimens from ani mals of the same species at successively younger ages, the intercellular substance becomes gradually more scanty, and finally altogether disappears, leaving the whole of the surface occupied by a cell-growth, which is a covering, but not an epithelium ; unless we extend the application of this objectionable word, and call the whole cartilage itself, what indeed we might with perfect truth, " a modified epithelium." The accuracy of this description of the cartilage of very young animals is easily veri fied by a vertical section ; and, if it be made sufficiently deep, it will include a portion of another structure, and a different process, with which it may be advantageous to com pare it. At the furthest extremity of such a
section, we see the ossification of temporary cirtilage actively going forward. First comes the formation of cancelli, and the enclosure of cells ; next, a little nearer the articular surface, the greatly dilated cells are arranged in closely-packed rows, the bottoms of which rest in cups of bone, which will soon become cancelli. Still approaching the articular sur face, we find the cartilage corpuscles smaller, more refractile, and flatter ; but yet with a distinctly linear arrangement. The loss of this arrangement in rows seems to indicate the limit of the ossifying cartilage and the commencement of the articular lamina; and I have often seen the distinction still further marked out by a horizontal fissure in this situation,—the effect of accidental violence, no doubt, but perhaps indicative of some deficiency of cohesion dependent on structure.
Immediately beyond this situation, the cartilage cells are scattered irregularly but closely through the transparent intercellular substance. They are angular and refractile, and they contain a large granular nucleus. Many of them are elongated, and somewhat spindle-shaped, while many more are tri angular ; and these two forms appear respec tively to precede and follow a fissiparous multiplication of their numbers, the constancy and accuracy of which would almost allow of its being termed a bisection. The details of this process have been already alluded to in speaking of the subtendinous bursw, and are too well known to need any recapitulation here. From hence onwards to the articular surface, the cells become more numerous, larger, and less angular in shape, until finally, on the surface itself, the increase of their number and size results in a continuous layer. But the appearances of this multipli cation are not seen in the most superficial stratum of all, although the prevalence of the hemispherical outline still indicates the binary nature of the fission ; whence it seems pro bable, that just upon the surface the increase is one of bulk only.
In fig. 403. is represented a vertical view of the superficial and of a deeper layer, which contrasts them in the particulars just men tioned. The condition of these cartilages in the adult fishes and reptiles closely resembles this description of their appearance in the young mammal, in the complete cellularity of their surface. For the knowledge of this fact, I am indebted to Mr. Quekett.