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FORMATION OF THE MARROW OF BONES. 409

in any way restricted to that of the original osseous rudiment, but often considerably exceeds it. Thus the development of bone, when taken as a whole, does not consist merely in the gradual apposition of a succession of fresh osseous layers derived from periosteum and cartilage, but also in the continual replacement of the innermost layers of the bone by masses of marrow.

In the interpretation of these facts the blastema theory was long appealed to as the great authority. Havers and Duhamel who made excellent investigations into the history of bone, started with the supposition that a nutritious juice (succus nutritius) was secreted, from which the new masses arose. The development of the marrow was imagined to consist in a formation of cavities, into which first a viscous juice and then a fatty matter was secreted— cavities which were invested by the medullary membrane, and whose contents varied with age. However, as I have already pointed out to you, there are no sacs in the areolae of the bones, but a continuous tissue,the medullary tissue, which fills the medullary spaces [cancelli] and cavities and belongs to the class of connective tissues, although it considerably differs from ordinary connective tissue. We have therefore here to deal, as you see from this simple fact, with a substitution of tissues. As osseous tissue1 is formed out of periosteum and cartilage, so marrow is formed from osseous tissue, and the development of a bone consists not merely in the formation of osseous tissue, but it presupposes that the series of transformations goes beyond the stage of bone, and that medullary tissue is then produced. Medullary tissue therefore constitutes in some sort the physiological termination of the formation of bone as an organ.

1 Osseous tissue (tela ossea, tissu osseux) = bone-corpuscles + calcified intercellular substance. Boue as an organ = osseous tissue+medullary tissue-fperiosteum+vessels+nerves. Osseous substance is sometimes taken to mean a portion of bone considered as an organ.—From a MS. note by the Author.

However simple this view may be, still it furnishes us with a picture of the growth and history of bone different from the traditional one. Formerly, observers nearly always contented themselves with viewing the matter much in the same light that osteologists are wont to do; they took a macerated bone, examined it when divested of all its soft parts, and built up the processes accordingly. It is, however, necessary that the relations should be traced in the moist, living healthy or diseased bone, and that one should pay attention not only to the development of bone upon the outside from the growing layers of the cartilage and periosteum, but also to that of the medulla on the inside, as the ultimate product of development in .this class of tissues, even if it be not the noblest one. The most important and really decisive point, through which the whole subject of bone acquires another aspect, is, I consider, this, that the bone in the formation of marrow is not simply dissolved and its place taken by some exudation or blastema, but that the dissolution of the osseous substance is a transformation of the osseous tissue, and that the dissolution results from a transformation of the intercellular substance of the bone into a soft mass of tissue which is no longer in a condition to retain the calcareous salts. If therefore you ask whence the new elements come which arise in the midst of osseous tissue, or how a cancer or collection of pus can form in the middle of the compact cortex of bone, I return you the very simple answer, that they arise in precisely the same manner, that in the course of the natural and normal development of bone the marrow arises. In no part does the osseous tissue first dissolve, then an exudation, and next a new-formation, follow, but the existing tissue is directly converted into the succeeding one. The existing osseous tissue is the matrix of the succeeding cancerous tissue, the cells of the cancer are the immediate descendants of the cells of the bone.

GROWTH OF CARTILAGE. 411

If now we consider the course of the formation of bone a little more in detail, we find, as we have already in part seen, that the cartilage prepares for ossification in such a way, that its cells in the first instance become larger; that divisions then take place in them, first in the nuclei and afterwards in the cells themselves; that these divisions then proceed with great rapidity, so that we obtain larger and larger groups of cells, and in a comparatively short time the place of a single cell is occupied by a relatively very large group of cells (Fig. 124). You will remember from my first lecture (p. 7), that a cartilage-cell is distinguished from most other cells by its secreting a special membranous capsule in which it is enclosed. This membranous capsule, on the division of the cells which it contains, sends in septa between them, which serve as new envelopes for the young cells, yet in such a way, that even the gigantic groups of cells, which proceed from each of the original cells, are still enclosed in the greatly enlarged parent capsule.

It is manifest, that the greater the number of cells which undergo this change, the larger the cartilage will become, and that the height to which any one of us attains, essentially depends upon the extent to which growth occurs in the individual groups of cartilage-cells. Whether we ultimately become tall or short, is, if I may say so, left entirely to the discretion of these elements.— When the growth of the proliferating cartilage has reached this point, the cellular elements are very close together, so that a comparatively trifling quantity of intercellular substance lies between them (Fig. 124). The farther the development advances, the more does the appearance of the cartilage alter, and at last it looks almost like densecelled vegetable tissue. The cells themselves however are difficult to be seen, because they are extremely sensitive; they readily shrivel up upon the addition of the mildest fluids, and then appear like angular and jagged corpuscles, almost analogous to those of bone, with which however they have at this time nothing to do.

[graphic]

The cells which have sprung from this excessive proliferation of the originally simple cartilage-cells, constitute the parent structures from which proceeds all that afterwards arises in the longitudinal axis of the bone, and especially the osseous and medullary tissue. The cartilagecells may be converted by a direct transformation into marrow-cells and continue as such; or they may first be

Fig. 126. Vertical section through the ossifying border of a growing astragalus, c. Cartilage with smallish groups of cells, p, the layer where the proliferation and enlargement are the most marked along the line of calcification. In the cartilage-cavities are seen, partly, complete nucleated cells, partly, shrivelled, angular and granular looking bodies (artificially altered cells). The dark mass advancing into the intermediate substance represents the deposition of calcareous salts, behind which the formation of medullary spaces (m, m, m) and osseous trabecula e [spicula] is here beginning with unusual rapidity. The marrow has been removed; round the cavities which lie farthest back, the trabecular are surrounded by a lighter border of young osseous tissue (produced from marrow). 300 diameters.

TRANSFORMATION OF CARTILAGE. 413

converted into osseous, and then into medullary, tissue; or lastly they may first be converted into marrow and then into bone. So variable are the permutations of these tissues in themselves so nearly allied, and yet in their external appearance so completely distinct. When a direct transformation into marrow is the first effected, the old intercellular substance of the cartilage at the border next to the bone begins first of all to grow soft; then some of the adjoining capsules usually also very soon experience this change, so that the cellular elements come to be more or less set free in a softer basis-substance. Simultaneously with the occurrence of this softening the chemical reaction of the tissue also becomes altered, and we always obtain the distinct reaction of mucin. At the same time divisions begin to take place, and this not in the same way as previously, when the cellular elements at once separated into two new analogous cells (hyperplasia), but rather in such a way, that a number of little nuclei arise in them (physiological heteroplasia). Subsequently, in proportion as this process of transformation reaches a higher and higher pitch, and fresh portions of the intercellular substance are continually being converted into this more homogeneous and soft matter, the cells generally divide, and we obtain a number of smaller ones, which are very minute in comparison to the large cartilage-cells, from which they proceeded, and contain either a single nucleus with a nucleolus, or sometimes also, like pus-corpuscles, several nuclei. Thus gradually arises a tissue extremely rich in cells, the young, red, medullary tissue, as we generally find it in the marrow of new-born infants. If the process stops here, the size of the transformed spot indicates at the same time the extent of the subsequent medullary space. Subsequently, these little cells may take up fat, and then it appears, first in small granules, but by degrees in large drops, and at last to such an extent that the cells are entirely filled with them. Thereby the

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