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calcareous matter has been deposited in the matrix in such a manner as to leave intervals of a form more or less stellate. The reason of this will presently appear. The calcareous matter is at first deposited so as to form a network with nearly equal meshes. Each space contains an oval mass of living germinal matter and is the earliest condition of a lacuna. (Fig. 43, d.)

The elementary parts concerned in the formation of lacunæ in the above specimen are represented in different stages of growth in figs. 44, 45, 46, under a power of 1700 diameters.

Fig. 51 shows the manner in which the earthy matter is deposited in the matrix of cartilage. It is copied from a section of the temporal bone of a frog. Globules of earthy matter may be seen to form imperfect rings around the cartilage cells. The calcareous matter is always deposited in the matrix (formed material), at a point midway between adjacent "cells" that is in the oldest portion of the formed material of the cartilage. The deposition gradually proceeds from without inwards. The outer part of the germinal matter of the cell gradually undergoes conversion into matrix, which in its turn becomes impregnated with calcareous matter until only a small space remains in which the nucleus still exists.

Figs. 50 to 55 show these stages, and in many specimens, especially from the frog, rounded globules of calcareous matter which coalesce and undergo great change in form, can be demonstrated without difficulty in lacunæ in an advanced state of formation (fig. 54). Mr. Rainey has watched this process and seems to consider that molecular alterations in the earthy particles are the essential changes to which the formation of bone is due.

I have examined the process of ossification as it occurs in various animals with the aid of carmine, and have always been able to demonstrate masses of germinal matter in a position

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corresponding to the lacunal space. I believe these masses of germinal matter to be as necessary to the production of bone as they are to the formation of every other tissue, and feel certain they are constantly present, and that through their agency alone, osseous, as well as all other tissues, is formed. They are not directly concerned in the deposition of the calcareous matter, but the matrix in which this is deposited cannot be formed without them, and it is probable that by their instrumentality alone the regular circulation of fluids holding the calcareous matter in solution is maintained, and thus the extreme regularity with which the growth of the tissue occurs is ensured.

For some time after the first deposition of the calcareous matter in the formed material, very thin fragments of the bone torn away exhibit the appearance of fibres (a fact pointed out many years ago by Dr. Sharpey), in the substance of which globules have been deposited (fig. 56), but slowly the calcareous matter becomes more homogeneous, in consequence, probably, of changes occurring in its substance, and its more perfect incorporation with the organic matrix, and ultimately the hard mass appears even in texture, uniformly transparent, and penetrated everywhere by minute tubes.

It seems to me that these tubes are the altered spaces which are left between the calcareous globules originally deposited. They were at first triangular in outline, but gradually they have become altered by the filling up of the angles, until at last they become pores, the section of which is nearly circular.

From appearances I have seen in some preparations of the bones of the frog's skull (frontal, parietal), I feel sure that in this case the bone results from changes in the original cartilage. The nucleus of the cartilage cell remaining as the nucleus in the lacuna. The calcareous matter deposited in the matrix around a cartilage cell undergoes changes, probably slowly becomes incorporated with the organic matter, and gradually ceases to exhibit the appearance of being composed of separate masses, and becomes more homogeneous. The spaces become

canaliculi, and the mass at last assumes the structure of perfect bone.

For some time separate calcareous particles are seen within the outline of the lacunæ, which gradually diminishes in size as calcareous matter is deposited in the matrix from without inwards. Fig. 54 represents about one-third of the inner part of a recently-formed lacuna of the frog, magnified 1,700 diameters. A part of the nucleus is seen in the lower part of the drawing.

In the development of the long bones of mammalia, on the other hand, it is equally certain that the spongy, imperfectly formed bone at first developed is gradually removed, and gives place to new bony tissue of a more perfect structure not formed from cartilage, while, as is well known, there are examples of the formation of bone without the existence of cartilage at any period, in the case of certain bones of the cranium.

During the occurrence of ossification in the cartilage of the temporal bone of the frog I have observed here and there one cartilage cell growing larger than the rest. The germinal matter dividing and subdividing into smaller masses which increase at the expense of the surrounding matrix. Thus a space filled with granular cells results, while around it the process of ossification proceeds.

The deposition of fatty matter in such granular cells is a subsequent process, and closely resembles the change which occurs in other elementary parts where fat is formed (see page 125). Fig. 42 shows the process as it occurs in cartilage. Medullary cells are, therefore, in certain cases the direct descendents of a cartilage cell. In the development of frog's bone the germinal matter of some of the cartilage cells continues as the germinal matter (nucleus) of the perfectly formed lacuna, while that of one here and there gives rise to the formation of the medullary cells.

In mammalia the nuclei of the original cartilage cells remain imbedded in the imperfect bone which is formed. Some

parts of this by subsequent processes similar to those described in page 128, be comeconverted into lacunæ, and thus spiculæ of bone result, while some of the nuclei multiply and absorb the calcareous matter immediately surrounding them, and thus spaces, or cancelli are formed filled with granular cells (medullary cells). The walls of the cancelli only last for a certain time. They are gradually encroached upon by the medullary cells, and the medullary cavity results.

Just as the germinal matter of a cartilage cell may under certain circumstances increase, and by division and subdivision form a number of small masses which grow at the expense of the surrounding matrix, forming at last a considerable cavity containing granular cells, so the germinal matter in a lacuna may give rise to the formation of a multitude of little masses which increase and produce a cavity of considerable size.

Fig. 47 represents lacunæ of the ordinary size in the bone of the kitten, and in fig. 48 the process above described is taking place. In a short time probably the two large lacunæ thus produced would run together forming a space filled with medullary cells.

This subject requires further investigation, for there is reason to believe that the exact conditions which determine why certain elementary parts become lacunæ, while here and there one gives rise to the production of granular cells which can never form anything higher than adipose tissue, may to a great extent be elucidated.

Having examined the preparations, let us now consider how the appearances observed accord with the views generally entertained with reference to the formation of lacunæ and canaliculi. Upon this question the widest differences of opinion exist.

Henle compared the formation of the lacunæ to the changes which occur in the walls of certain vegetable cells through the secondary deposits of which pores are left.

Kölliker considers that the capsule of the cartilage cell and the matrix, become impregnated with calcareous matter, while the granular cell corresponding to the primordial utricle of the vegetable cell and with the endoplast of Mr. Huxley, remains within unaltered. He thinks that the canaliculi extend through the matrix by resorption.

Virchow regards bone as consisting of cells and an intercellular substance, and he considers the canaliculi to be processes which grow from the cells.

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In the following MS. note, copied from page 417 of Dr. Chance's translation, he expresses himself very clearly as to the manner in which processes are formed from cells. cartilage cells (and the same holds good of the marrow cells), during ossification throw out processes (become jagged) in the same way that connective tissue corpuscles, which are also originally round, do, both physiologically and pathologically. These processes which in the case of the cartilage cells are generally formed after, but in that of the marrow cells frequently before, calcification has taken place, bore their way into the intercellular substance, like the villi of the chorion do into the mucous membrane and into the vessel of the uterus, or like the Pacchionian granulations (glands) of the pia mater of the brain into (and occasionally through) the calvarium." Again, "The cells which thus result from the proliferation of the periosteal corpuscles are converted into bone corpuscles exactly in the way I described when speaking of the marrow. In the neighbourhood of the surface of the bone the intercellular substance grows dense and becomes almost cartilaginous, the cells throw out processes, become stellate, and at last the calcification of the intercellular substance ensues."

There are few points in minute anatomy upon which such different views have been advanced as the one under consideration, and you cannot fail to notice that observers differ not only in the explanations and opinions they have put forward, but

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