B. PERMANENT cartilages are met with under two forms : 1, the articular, attached to bone, and entering into the formation of joints ; 2, the non-articular, forming canals more or less per fectly.
I. The articular cartilages are called diar throdial, obducent, or of incrustation, when they belong to the moveable articulations; synarthrodial when connected with those very limited in their motions, or the immoveable articulations of some authors. We think it unnecessary to do more than refer to these cartilages here, as their characters will be found fully described in the article ARTICULATION.
H. The non-articular cartilages are usually much more flexible than the articular. In some cases they are attached to bones, and lengthen them out, as the preceding class. Of this we see examples in the nose, the auditory canal, and the Eustachian tube. In other cases they are insulated, forming the basis of distinct organs, as the larynx, the trachea, the eyelids. All the cartilages of this class have a well marked perichondrium.1' Some of them, as the epiglottis, the tarsal cartilages, and those of the alze nasi, are so thin, so flexible, and assume so much of a fibrous appearance from their perichondrium, that Bichat placed them amongst the fibro-cartilages ; but these last never have perichondrium, and their fibrous texture is distinctly independent of their investment, as is easily seen without any preparation. (See FIBRO-CARTILAGE.) The structure of non-articular cartilage, like the other forms, may, by protracted maceration, be shown to be fibrous ; but the arrangement of its fibres is different ; they interlace a good deal more.
The physical properties of cartilages are such as to fit them admirably for the functions which they have to perform. They are solid, resisting, and incapable of extension, that they may be able to preserve the form of certain parts as effectually as bone; and they are flexible and elastic, to enable them to yield in some degree, and immediately to resume their original shape.
Elasticity is the property most essential to them, and on this their usefulness mainly de pends. Its existence is easily demonstrated. lf the blade of a knife be pressed into a diar throdial cartilage, the reaction of the displaced fibres expels it with force ; and a piece of any cartilage, if bent between the fingers, returns with a spring to its former shape. The elastic fibres of diarthrodial cartilage are so placed as to receive impressions on their extremities ; they yield a little to force, and only a little, else the ligaments would be too much relaxed ; but they yield enough to let the opposite sur faces accommodate themselves to each other, and to deaden the shocks which would other wise have an injurious effect on the nervous centre. In fact, these articular cartilages serve as a series of springs between the ground and the delicate organs which they support. The elasticity of the costal cartilages is obvious and essential. They are subject to torsion in the act of inspiration, and by their reaction become an important agent in expiration.
Differences depending upon age.—Cartilages are soft, transparent, and like jelly in the very young fcetus. Gradually, as the individual advances to maturity, they become opaque, white, firm, and elastic ; and in the adult these qualities are in their greatest perfection. In
old age they lose again their elasticity. and flexibility ; a yellowish colour takes the place of their beautiful pearly white; they become dry and brittle, and shew a great tendency to ossify.
Organization. — Cartilage appears at first sight to be perfectly homogeneous throughout, like a concrete jelly, not shewing any traces of organization, nor exhibiting the least appear ance of vessels. But, as an attentive examina tion proved it to be fibrous, so we shall be able to satisfy ourselves that it possesses an organi zation similar to other parts of the living sys tem. In healthy cartilage, it is true, no red vessels can be demonstrated, neither can the finest injection be made to penetrate it, nor will madder used in food colour it. But dis ease sometimes shows red vessels ramifying through its substance .* and several other phe nomena lead us to the conviction that it is at all times permeated with vessels, though they may be too fine to admit the red globules. For instance, we find cartilage assume a yellow tinge in jaundice. If we slice off a bit, the dry surface is soon moistened with a serous fluid, which, doubtless, comes from its colourless vessels. Exposed cartila„0-es have been known to granulate, which implies the existence of vessels, and perhaps of cellular substance. And we know that in the old and laborious there is often not the least sign of wear, although the enamel of the teeth be quite worn away. Where a perichondrium is present, we may suppose the vessels first ramify in it before they enter the cartilage. Dr. William Hunter describes the arrangement of the vessels which supply diarthrodial cartilage to be very peculiar. He says, "All around the neck of the bone there are a great number of arteries and veins which ramify into smaller branches, and communicate with one another by frequent anastomoses, like those of the mesentery. This might be called the circulus articuli vasculosus the vascular border of the joint. The small branches divide into still smaller ones upon the adjoining- sur face, in their progress towards the centre of the cartilage. We are seldom able to trace them into its substance, because they terminate ab ruptly at the edge of the cartilage, like the vessels of the albuginea oculi when they come to the cornea."1 It does not appear that nerves or absorbents have ever been traced into cartilages ; but the phenomena of disease, pain, ulceration, &c., convince us that they are supplied with both. Even in their healthy condition, though their animal sensibility is exceedingly low, scarcely perceptible, yet it probably does exist, and will manifest itself whenever any cause is operating upon them which might destroy their texture. We may, indeed, cut an exposed car tilage 'without pain, and the violent pressure it undergoes in a sound joint is unheeded. But the former is a kind of injury from which car tilage may be said to be totally exempted, and the latter is that for which it is peculiarly adapted. In either case sensibility would be useless or inconvenient. Let but a foreign body however get into a joint, between its cartilages, such as might disorganize them, and then an alarm is set up too great to be attributed to the synovial membrane alone, and depending, we may suppose, -in part at least, on the cartilage itself.