buried are often found in this condition; their form and position being retained until they are exposed to the air, or are a little shaken, when they crumble to dust. The proportion of the earthy matter of Bones to the animal basis may be differently stated, according as we include, in our estimate of the latter, the contents of the medullary cavity, the Haversian canals, and the cancelli, or confine ourselves to that portion only of the animal matter which is united with the calcareous element in the proper osseous tissue. According to the recent experiments of Dr. Stark, the relative amount of the two elements, in the latter estimate, is subject to very little variation, either in the different classes of animals, or in the same species at different ages, the animal matter composing about one-third, or 33 per cent., and the mineral matter two-thirds, or 663 per cent. The degree of hardness of bones does not altogether depend, therefore, on the amount of earthy matter they may contain; for the flexible, semi-transparent, easily-divided bones of Fish contain as large an amount of animal matter, as the ivory-like leg-bones of the Deer or Sheep. The usual analyses of Bone, however, have been made upon the former kind of estimate: and they show that the proportion of the earthy matter to the whole of the animal substance. contained in bone varies much in different animals, in the same animal in different ages, and even in different parts of the same skeleton. The reason of this will be apparent, when the history of the growth of Bone has been explained; since there is a gradual filling up of all the cavities at first occupied by fat-cells, vessels, &c., which does not cease with adult age, but which continues during the whole of life. In this manner the bones of old persons acquire a high degree of solidity, but they become brittle in proportion to their hardness. From the same cause, the more solid bones contain a larger proportion of bone-earth than those of a spongy or cancellated texture; the temporal bone, for example, containing 634 per cent., whilst the scapula possesses only 54 per

In the former of these bones, the proportion is nearly the same as that which exists in pure osseous tissue, the amount of the remaining tissues which it includes being very small, on account of the solidity of the bone; but the latter contains in its cancelli a large quantity of blood-vessels, fat-cells, &c., which swell the proportion of the animal matter from 33 to 46 per cent.

299. The Lime of bones is for the most part in a state of Phosphate, especially among the higher animals; the remainder is a carbonate. In Human bones, the proportion of the latter seems to be about onesixth or one-seventh of the whole amount of bone-earth. In the bones of the lower animals, however, the proportion of Carbonate is greater; and it is curious that in callus, exostosis, and other irregular osseous formations in the higher animals, the proportion of the Carbonate should be much greater than in the sound bone. In caries, however, the proportion of the Carbonate is less than usual. The composition of the Phosphate of Lime in Bones, is somewhat peculiar; eight proportions of the base being united with three of the acid. According to Professor Graham, it is to be regarded as a compound of two tribasic

* Edinburgh Medical and Surgical Journal, April, 1845.

phosphates; one atom of the neutral phosphate (in which one proportional of the acid is united with two of lime and one of water), being united with two proportionals of the alkaline phosphate (in which one part of acid is united with three of the base), together with an atom of water, which is driven off by calcination. Besides these components, some Chemists assert that a small quantity of Fluoride of calcium is present in Bone; but this is rather doubtful, since it has been shown by Dr. G. O. Rees that the solvent action upon glass, which has been supposed to be characteristic of fluoric acid, may be imitated by phosphoric acid in combination with water, which, if heated upon glass of inferior quality until it volatilizes, will act upon it with considerable energy. Other saline matters, such as phosphate of magnesia, oxides of iron and manganese, and chloride of sodium, are found in bones in small amount.

300. The first development of Bone is usually preceded by the formation of a Cartilaginous structure, which occupies the place afterwards to be taken by the bone; and it is commonly considered that the bone is formed by the calcification of the cartilage-substance. This, however, does not appear to be the case, as will be presently shown; and it would probably be more correct to say that the cartilage is superseded by bone. Moreover, Bone is frequently developed in the substance of Fibrous membranes; and the structure produced by this intramembranous ossification cannot be distinguished from that which is generated by the intra-cartilaginous.-We shall commence the history of the development of Bone, with the period in which its condition resembles that of the permanent Cartilages. As already mentioned, there is no essential difference between the temporary and permanent Cartilages, in regard to their ultimate structure; the former, however, are more commonly traversed by vessels, especially when their mass is considerable. These vessels, however, do not pass at once from the exterior of the cartilage into its substance; but they are conveyed inwards along canals, which are lined by an extension of the perichondrium or investing membrane, and which may thus be regarded as so many involutions of the outer surface of the cartilage. These canals are especially developed at certain points, which are to be the centres of the ossifying process; of these puncta ossificationis, we usually find one in the centre of the shaft of a long bone, and one in each of its epiphyses; in the flat bones there is one in the middle of the surface, and one in each of the principal processes. Up to a late stage of the ossifying process, the parts which contain distinct centres are not connected by bony union, so that they fall apart by maceration; and even when they should normally unite, they sometimes remain separate,-as in the case of the Frontal bone, in which we frequently meet with a continuation of the sagittal-suture down the middle, dividing it into two equal halves, which have originated in two distinct centres of ossification. It is interesting to remark that, in the two lowest classes of Vertebrata,-Fishes and Reptiles,-we find the several parts of the osseous system presenting in a permanent form, many of the conditions which are transitory in the higher; thus the different portions of each vertebra, the body, lateral arches, spinous and transverse processes, &c., which have their

distinct centres of ossification, but which early unite in Man, remain permanently distinct in the lower Fishes; the division of the frontal bone, just adverted to, is constant amongst Fishes and Reptiles; and in these classes we meet with a permanent separation of the parts of the occipital and temporal bones, which, being formed from distinct centres. of ossification, are at first distinct in the higher animals.

301. During the formation of the punctum ossificationis, and the spread of the vessels into the cartilaginous matrix, certain changes are taking place in the substance of the latter, preparatory to its conversion into bone. Instead of single isolated cells, or groups of two, three, or four, such as we have seen to be characteristic of ordinary Cartilage (§ 267), we find, as we approach the ossifying centre, clusters made up of a larger number, which appear to be formed by a continuance of the same multiplying process as that already described (Fig. 50). And

Section of Cartilage near the seat of ossification; each single cell having given birth to four, five, or six cells, which form clusters. These clusters become larger towards the right of the figure, and their cells more numerous and larger; their long diameter being 1-1500th of an inch.

when we pass still nearer, we see that these clusters are composed of a yet greater number of cells, which are arranged in long rows, whose direction corresponds with the longitudinal axis of the bone; these clusters are still separated by intercellular substance, and it is in this, that the ossific matter is first deposited (Fig. 51). Thus if we separate the

The same cartilage at the seat of ossification: the clusters of cells are arranged in columns; the intercel lular spaces between the columns being 1-3250th of an inch in breadth. To the right of the figure, osseous fibres are seen occupying the intercellular spaces, at first bounding the clusters laterally, then splitting them longitudinally and encircling each separate cell. The greater opacity of the right hand border is due to a threefold cause, the increase of osseous fibres, the opacity of the contents of the cells, and the multiplication of oil-globules.

cartilaginous and the osseous substance at this period, we find that the ends of the rows of cartilage-cells are received into deep narrow cups of

bone, formed by the transformation of the intercellular substance between them. Immediately upon the ossifying surface, the nuclei, which were before closely compressed, separate considerably from one another, by the increase of material within the cells; and the nuclei themselves become larger and more transparent. These changes constitute the first stage of the process of ossification, which extends only to the calcification of the intercellular substance; in this stage there are no bloodvessels directly concerned. The bony lamellæ thus formed, mark out the boundaries of the cancelli and Haversian canals, which are afterwards to occupy a part of the space that is hitherto filled by the rows of cartilage corpuscles.

302. Up to this point, there is no essential difference in the accounts of those who have most carefully studied the process of ossification; but in regard to the history of its subsequent stages, there is much discrepancy; and this especially with respect to the origin of the bone-lacunæ, which some regard as metamorphosed cartilage-cells, others as the spaces originally occupied by their nuclei, whilst others do not regard them as in any way derived from the cartilage-cells, but consider them as a new formation. Much may doubtless be urged in favor of each view; the author's own observations incline him to the latter, and lead him to regard the lacunæ as cells, which like the pigment-cells of Batrachia, &c., have sent out the stellate prolongations that constitute the canaliculi. All stages of gradation may be traced, between simple rounded cavities,-whose correspondence in size with the cells that are scattered in the midst of the consolidating blastema leaves scarcely any doubt of their identity with these,-and the lenticular lacuna with numbers of canaliculi proceeding from it. These gradations are particularly well seen during the process of ossification; so that it seems probable that the radiating extension of the cells takes place during the consolidation of the surrounding tissue.—It is an additional argument against the idea that the bone-lacunæ in any way originate from the cartilage-cells, that they are found to present exactly the same characters in bone which is developed in the substance of fibrous membrane (after the manner to be presently described), and in the formation of which, therefore, cartilage has had no participation.

303. Although, in a large proportion of the skeleton, the formation of Bone is thus preceded by that of cartilage, yet such is by no means invariably or necessarily the case; for the flat bones, such as the scapula, and those forming the roof of the skull, have usually only a centre of cartilage, beyond which the ossifying process extends in membrane only. This membrane is chiefly composed of fibrous fasciculi, corresponding with those of the white fibrous tissues; but amongst these are seen numerous cells, some about the size of blood-discs, but others two or three times larger, containing granular matter; and a soft amorphous or faintly granular matter is also found interposed amidst the fibres and cells. The process of ossification here seems essentially to consist in the consolidation of the fibres by earthy matter; for the first bony deposit is seen as an irregular reticulation, very loose and open towards its edges, and there frequently presenting itself in the form of distinct spicula, which are continuous with fasciculi of fibres in the surrounding

membrane. The limits of the calcifying deposit may be traced by the opaque and granular character of the parts affected by it; and it gradually extends itself, involving more and more of the surrounding membrane, until the foundation is laid for the entire bone. Everywhere the part most recently formed consists of a very open reticulation of fibrocalcareous spicula, whilst the older part is rendered harder and more compact by the increase in the number of these spicula. As the process advances, and the plate of bone thickens, a series of grooves or furrows, radiating from the ossifying centre, are found upon its surface; and these, by a further increase in thickness, occasioned by a deposit of ossific matter all around them, are gradually converted into close canals (the Haversian), which contain blood-vessels, and are lined by processes of the investing membrane. The lacunæ and canaliculi seem to take their origin in the cells which are interspersed among the fibres, their prolongations extending themselves, and insinuating themselves through the spaces left between the interlacing fibres, whilst the process of calcification is going on.

304. The first osseous tissue which is formed by either of these processes, has an irregular cancellated structure, analogous to that which is found at the extremities of the long bones in adults. This is gradu ally modified by changes which essentially consist in absorption and new deposition; for the absorptive process first unites minute areola into larger ones, by removing their partitions; and it is upon their interior walls that new osseous lamellæ are now deposited, from materials supplied by the blastema they contain. It is by a process of this kind, that the central medullary cavity is first formed in the bones of young animals. At an early period, no such cavity exists, and its place is occupied by small cancelli; this is the permanent condition of the bones in most Reptiles. The cancelli gradually enlarge, however; and those within the shaft coalesce with one another until a continuous tube is formed, around which the cancelli are large, open, and irregular. At the same time, the diameter of the surrounding shaft is increasing by the process of interstitial growth just described; so that the size of the medullary cavity at last becomes greater than that of the whole shaft. when its formation commenced. The aggregation of the osseous matter in a hollow cylinder, instead of a solid one, is the form most favorable to strength, as may be easily proved upon mechanical principles. The same arrangement is adopted in the arts, wherever it is desired to obtain. the greatest strength with a limited amount of material.

305. The growth of Bones takes place by the addition of new tissue to the part already formed; but this addition may take place in three modes, namely, by the development of new bone in the cartilage yet remaining between the different centres of ossification; by the development of new bone in the membrane covering the surface; and by the interstitial formation of new layers within the Haversian canals and cancelli of the part already formed, by which the requisite solidity is given to it. Of the first process we have the most characteristic example in the increase in length of a long bone, by the ossification of the cartilage which intervenes between the shaft and the epiphyses, and which continues to grow, up to the time of the final union of these