OF THE MARROW.
This is of an oily nature. It hardens when cold, in herbaceous animals ; but it remains fluid in those which are carni vorous. It has a reddish and bloody ap pearance in young animals; but this soon goes off. It is contained in fine membra nous cells, which do not communicate with each other. The marrow occupies the tube left in the middle of the long limes, and also fiUs the cancelli of their extremities.
The cellular substance, which contains the marrow, being condensed upon the inside of the walls of the bone, and adhe ring to them, has been termed the petit* teuni iniernum.
We observe in the principal bones arte ries, much larg-er than those which nou rish the bone, penetmfing these bodies ob liquely, and spreading their branches upon the medullary cells.
Various unsatisfactory opinions have been proposed concerning the use of the inarrow. The utility of the bones be nig formed as they are, small and tubular in the middle, expanded and spongy at their extremities, has been already ex plained. If then spaces are necessarily left in their interior parts, those spaces must be filled with something ; for they cannot be left void, or the immense pres sure of the atmosphere would crush their sides, and destroy- 1.11C VICOUM. TI1CFC 1S 110 matter in the animal body more suit able to fill their spaces than the marrow ; and it is to be regarded as a part of the adipous system of the animal.
From the circumstances which have been detailed in the foregoing account, -ciz. the great and general vascularity of banes; the quantity of soft substance ex isting in every part of them ; their growth and mutation of form in disease, it is natural to conclude, that there exist in Is a semipellueid substance, of a milk white or pearly colour, entering into the composition of several parts of the body. It holds a middle rank, in point of firm ness, between bones, or hard parts, and the softer constituents of the human frame. It appears, on a superficial ex amination, to be homogeneous in its tex ture ; for, when cut, the surface is uni form, and contains no visible cells, cavi ties, nor pores; but resembles the section of a piece of glue. It possesses a very
high degree of elasticity; which property distinguishes it from all other parts of the body. Hence it enters into the coinpo sition of parts, whose functions require the combination of firmness with pliancy and flexibility: the preservation of a cer tain external form, with the power of yielding to external force or pressure.
Cartilages are covered by a membrane, resembling-, in texture and appearance, as well as 111 its office, the periostcum of bones ; this is termed the perichondrium. They receive arteries and veins from this membrane : these vessels, however, have never been demonstrated in the cartila ginous crusts of articular surfaces. Ab sorbent vessels cannot be actually' shewn, hut their existence is abundantly proved by many' phenomena. The conversion of cartilage into bone is alone sufficient for this purpose. The cartilaginous sub stance is gradually- removed, as the for mation of the bone advances. In affec tions of the joints, their cartilaginous co verings are often both entirely destroyed, or partially removed which appearances can only be ascribed to the action of ab sorbent vessels It does not seem to possess nerves, as it is entirely destitute of sensibility.
The thinner cartilages of the body are resolved by maceration into a kind of fibrous substance : e. g. those of the or gans of sense. Those of the ribs are found by long maceration to consist of concentric oval lamina. In some there are tendinous fibres intermixed ; as in those of the rertebrze.
Anatomists divide cartilages into two kinds: the temporary and the permanent. The former are colifined to the earlier stages of existence : the latter common ly retain their cartilaginous structure throughout every period of life.
The temporary cartilages. are those in which tbe bones of the body are formed. They are hence called by the Latin wri ters meacentes. All the bones of the body, except the teeth, are formed in a nidns of cartilage. The form of the bone, with its various processes, is accurately repre sented in these cartilaginous primordia ; and it is the substance alone which changes.