epiphyses thus formed are separated, as long as growth continues, from the shaft or diaphysis by an intervening portion of cartilage, which is at last ossified, and the bone is then consolidated. A remarkable exception to the ordinary mode of ossification of the cartilage-bones occurs in the terminal phalanges of the digits. In these the calcification of the cartilage begins at the distal extremity or tip, and the sub-periosteal deposit appears simultaneously at the same point, and forms a cap-like expansion over the end of the phalanx. The irruption of the osteoblastic tissue also first occurs at this place. The expanded portion of the phalanx which bears the nail is formed independently of cartilage (Dixey). Growth and absorption of bone.-The time of final junction of the epiphyses is different in different bones; in many it does not arrive until the body has reached its full stature. Meanwhile, as above described, the bone increases in length by the ossification continuing to extend into the intervening cartilage, which goes on growing at the same time; and it appears that in the part of the shaft already ossified little or no elongation takes place by interstitial growth. This is shown by an experiment first made by Hales and afterwards by Duhamel and by John Hunter, in which, two or more holes being bored in the growing bone of a young animal at a certain measured distance from each other, they are found after a time not to be farther asunder, although the bone has in the meanwhile considerably increased in length. On the other hand, if one hole be bored in the epiphysis and another in the shaft, they become distinctly removed from one another with the growth of the bone. Moreover, it is well known that if the intervening cartilage in growing bone be injured by disease or removed by the knife, the growth of the bone. in length permanently ceases. Both Hales and Duhamel in experimenting on the growing tibia of a chicken, observed that the elongation was much greater, at the upper end. Humphry has shown that in the arm-bones the elongation is greater at the end furthest from the elbow joint, and in the leg-bones at the end which is next the knee joint. In the human subject between the first and the fourth or fifth years, the long bones grow chiefly in length, scarcely at all in thickness. The shaft of a long bone increases in circumference by deposition of new bone. on its external surface, while at the same time its medullary canal is enlarged by absorption from within. This can be determined by two methods of experimenting. Thus, in the first place, a ring of silver or platinum put round the wing-bone of a growing pigeon, becomes covered with new bone from without, and the original bone. included within it gets thinner, or, according to Duhamel, who first made the experiment, is entirely removed, so that the ring comes to lie within the enlarged medullary canal. Secondly, madder given to an animal along with its food tinges those parts in which deposition of new bone is taking place. The earth of bone appears to act as a sort of mordant, uniting with and fixing the colouring matter; and, as in this way the new osseous growth can be readily distinguished from the old, advantage was taken of the fact by Duhamel, and afterwards by Hunter, in their inquiries as to the manner in which bones increase in size. By their experiments it was shown that when madder is given to a young pig for some weeks, the external part of its bones is deeply reddened, proving that the new osseous matter is laid on at the surface of that previously formed. Again, it was found that, when the madder was discontinued for some time before the animal was killed, an exterior white stratum (the last formed) appeared above the red one, whilst the internal white part, which was situated within the red, and had been formed before any madder was given, had The occurrence of a certain amount of interstitial expansion is strenuously upheld by J. Wolff (see Bibliography). become much thinner; showing that absorption takes place from within. In this last modification of the experiment also, as noted by Hunter, a transverse red mark is observed near the ends of the bone, beyond which they are white; the red part indicating the growth in length during the use of the madder, and the white beyond, that which has taken place subsequently,-thus showing that the increase in length is caused by the addition of new matter to the extremities. Madder administered while the process of formation of the concentric lamellæ of the Haversian systems is going on, colours the interior and recently-formed laminæ, so that in a cross section the Haversian apertures appear surrounded with a red ring. Flourens, and more recently, Kölliker, have repeated and varied these experiments, and have represented the results in beautiful delineations. Kölliker has, in addition, carefully investigated the microscopic appearances observed in the process of absorption of bone. From the results of his researches (which were in part anticipated by those of Lovén), it would seem that the process is essentially dependent on the presence of large multi-nucleated cells, by him termed "ostoclasts," identical with the "myeloplaxes" of Robin (see p. 267), which excavate, in Fig. 320.-THREE OSTOCLASTS FROM ABSORPTION SURFACES OF GROWING BONE. 400 DIAMETERS Fig. 321.-BONY TRABECULA FROM THE LOWER JAW OF A CALF EMBRYO WITH HOWSHIP'S FOVEOLE AND GIANT-CELLS AT THE ENDS WHERE ABSORPTION IS PROCEEDING AND OSTEOBLASTS COVERING THE SIDES WHERE BONE IS BEING DEPOSITED (Kölliker). the part which is undergoing absorption, small shallow pits (foveola) in which also they lie. These pits were first noticed by Howship: they seem to occur wherever absorption is proceeding, and it is to them that the festooned appearance of the Haversian spaces (p. 264) is due. The ostoclasts (figs. 320, 321) vary in size, but are always many times larger than a blood-corpuscle: in shape they are spheroidal or flattened, with either an even or an irregular outline. Their substance is granular in appearance, and they each contain from two to ten clear round nuclei, but this number may be considerably exceeded, whilst on the other hand, there may be but one large nucleus provided with a number of bud-like projections. The ostoclasts have frequently on the side by which they are in contact with the bone a thickened striated border (fig. 320, a), somewhat similar to the well-known thickened base of the columnar epithelium-cells of the intestine. With respect to the origin and destiny of the ostoclasts, they are regarded by Kölliker both as in the first instance derived from and as eventually breaking up into osteoblasts. Ostoclasts are found in connection with the roots of the milk teeth where these are undergoing absorption to make way for the permanent set. They were also noticed by Billroth to produce absorption of ivory pegs which had been driven into bone. Precisely similar cells occur under pathological conditions in various situations apart from any hard tissue, and have long been known as "giant-cells" (Riesenzellen, Virchow). The changes of shape which the bones undergo in the process of growth, as well as any changes which may occur in them in adult life, are all produced in the same manner as the increase of size that is to say, not by interstitial growth and expansion of the substance of the bone in one direction more than in another, but by a deposition of new bone by osteoblasts at some parts and a simultaneous absorption by ostoclasts at others; whilst in other places again neither absorption nor deposition is occurring-just as a modeller corrects his work by laying clay on at one part whilst removing it at another.1 Since during the growth of bones their shape is becoming continually altered, it follows that in nearly all bones during growth there are parts of the bone which are in process of absorption, and others which are in process of more active deposition than the rest. In most of the long bones, towards their ends, absorption is generally taking place at one side, and deposition on the opposite side. The former process may, and probably does, proceed to such an extent, that the endochondral bone may be laid bare or even partially absorbed, but after a while, when the absorption has ceased at any part, re-deposition may take place, the ostoclasts being replaced by osteoblasts, and successive circumferential lamellæ being deposited by these. A large amount of variation is met with in the different bones of the skeleton in the relative extent to which they are formed in cartilage and in the sub-periosteal tissue respectively. Whereas in some, such as the shafts of the long bones of the limbs, the endochondral bone is almost entirely removed, as we have seen, and periosteal bone substituted for it; in others, such as the bodies of the vertebræ, and the epiphyses of the long bones, a much larger proportion of the adult bone has had an endochondral formation. In one or two bones or parts of bones again, which may be said to have typically an intramembranous origin, cartilage may, according to Kassowitz, become developed under the periosteum at certain places, and the continuation of the ossification may occur in this secondarily developed cartilage. This is said to be the case with the clavicle, the foundation of which is laid in membrane, but which is found at a later period to have cartilaginous ends; and also with the halves of the lower jaw-bone, which develops cartilaginous ends both towards the symphysis and towards the articular and coronoid processes, these cartilaginous ends being altogether distinct from the cartilage of Meckel, which at those parts is unconnected with the jaw-bone, although at another place (in front) it is involved in the ossification of the maxilla (see vol. ii., p. 78). Kassowitz has described similar cartilaginous developments in connection with the sub-periosteal tissue at the tuberosity of the radius and the spine of the scapula. They are merely an extension of the process which normally goes on at the ossification groove (p. 279). The time of commencement of ossification in the different bones, as well as the number and mode of conjunction of their centres of ossification, are treated of in the Descriptive Anatomy (vol. ii.). Regeneration of bone. In the reunion of fractured bones, osseous matter (often preceded by a new formation of cartilage), is formed between and around the broken ends, connecting them firmly together; and when a portion of bone dies, a growth of new bone very generally takes place to a greater or less extent, and the dead part is thrown off. The importance of the periosteum in the process of repair is shown by the fact that if a portion of periosteum be stripped off, the subjacent 1 For special details of this modelling process as it is met with in the different bones of the skeleton, the reader is referred to Kölliker's memoir; Die normale Resorption des Knochenge webes. Leipzig, 1873; and to a paper by Kassowitz (Die normale Ossification, &c.) in Stricker's Med. Jahrb, 1879-1880. bone will be liable to die and exfoliate; conversely, if a large part or the whole of a bone be removed and the periosteum at the same time be left intact, the bone will, in a great measure, be regenerated. Osseous formation will even occur in connection with portions of periosteum which have been stripped away from the bone itself and intertwined amongst the muscles of the part, or even with portions that have been entirely removed from a bone and transplanted to a soft tissue (Ollier). It is doubtful if the marrow-tissue can assist in the regeneration of bone. Experiments which have been made to determine this point would seem to show that although in the young bone, where the osteoblasts still retain their osteogenic function, the medullary tissue may take an active part in the formation of the first-formed new bone or " callus," in the adult no such participation of the marrow in the regeneration of bone takes place. In the young subject even small pieces of the bone itself can be transplanted, and McEwen has succeeded in renewing the greater part of the excised humerus of a child by the introduction, at successive periods, of portions of fresh bone removed from another patient. It was long supposed that all the bones of the skeleton were preceded by and deposited in cartilage. Nesbitt, however, showed in 1736 that some of the flat bones were formed independently of cartilage, and he further maintained that the cartilage is “entirely destroyed; he therefore considered it to be a mere temporary substitute; but the steps of the process of intracartilaginous ossification as now traced with the aid of the microscope were unknown to him, and it was not until the year 1846 that the manner of formation of bone and the extensive replacement of the primarily ossified cartilage by new bone formed in membrane was made clear by the researches of Sharpey, who published the results of his work in the fifth edition of this book. RECENT LITERATURE. Bajardi, D., Ueber die Neubildung der Gelenkenden nach der subcapsulo-periostalen Resection, Moleschott's Unters. zur Naturlehre, xiii.; Ueber die Neubildung von Knochensubstanz in der Markhöhle und innerhalb der Epiphysen und über die Regeneration des Knochenmarks in den Röhrenknochen, Ibid., 1883. Bizzozero, Ueber die Atrophie der Zellen des Knochenmarkes, Arch. f. mikr. Anat., 33, 1889; Neue Untersuchungen ueber d. Bau des Knochenmarkes bei den Vögeln, Arch. f. mikr. Anat., 35, 1890.1 Bonome, A., Zur Histogenese der Knochenregeneration, Virchow's Archiv, Bd. 100, 1885. Brösike, G., Ueber die sogenannten Grenzscheiden des Knochenkanalsystems nebst Bemerkungen über die Keratinsubstanzen, Archiv f. mikrosk. Anat., Bd. 26, 1885. Demarbaix, H., Division et dégénérescence des cellules géantes de la moelle, La Cellule, t. v., 1889. Denys, J., Sur la structure de la moelle des os et la genèse du sang chez les oiseaux, La Cellule, t. iv., 1888. Ebner, v., Sind die Fibrillen des Knochengewebes verkalkt oder nicht? Archiv f. mikrosk. Anat., Bd. 29, 1887. Egger, G., Experimentelle Beiträge zur Lehre vom interstitiellen Knochenwachsthum, Virchow's Archiv, Bd. 99, 1885. Fargerlund, On the development of ossification-points during the first year of life, London Medical Recorder, 1890. Geelmuyden, H. C., Das Verhalten des Knochenmarks in Krankheiten und die physiologische Function desselben, Virchow's Archiv, Bd. 105, 1886. Howell, W. H., Observations upon the occurrence, structure, and function of the giant-cells of the marrow, Journal of Morphology, iv., 1890. Kölliker, A., Der feinere Bau des Knochengewebes, Zeitschr. f. wissensch. Zool., Bd. xliv., 1886 ; Nachwort zu meinem Artikel " Ueber den feineren Bau des Knochengewebes," Zeitschr. f. wiss. Zoologie, Bd. xlv., 1887. Leser, E., Ueber histologische Vorgänge an der Ossificationsgrenze mit besonderer Berücksichtigung des Verhaltens der Knorpelzellen, Arch. f. mikrosk. Anat., 32, 1888. Lilienberg, J., Beiträge zur Histologie des Knochengewebes, Mémoires de l'acad. imp. d. Sciences de St. Petersb., t. xxxiii., 1885. Petroni, Istologia della polpa del midollo osseo rosso, &c., Anat. Anzeiger, 1889. 1 Other recent papers by Bizzozero are referred to in the literature relating to the development of blood-corpuscles. VOL. I. U Pommer, G., Ueber die Ostoklastentheorie, Virchow's Arch., Bd. 92, 1883; Untersuchungen über Osteomalacie und Rachitis nebst Beiträgen zur Kenntniss der Knochenresorption und -Apposition in verschiedenen Altersperioden, Leipzig, 1885. Schaffer, J., Die Verknöcherung des Unterkiefers und die Metaplasiefrage, Archiv f. mikr. Anatomie, Bd. 32, 1888; Ueber d. feineren Bau fossiler Knochen, Wiener Sitzungsb., 98, 1889. Schmid-Monnard, C., Die Histogenese des Knochens der Teleostier, Zeitschr. f. wissensch. Zool., Bd. 39, 1883. Smith, E. Herbert, Enthalten die Knochen Keratin Zeitschrift für Biologie, xix., 1884. Stricht, O.v.d., Recherches sur la structure fondamentale du tissu osseux, Arch. de Biologie, ix. Tafani, A., Le tissu des os, les fibres perforantes ou de Sharpey, Archives ital. de biologie, viii., 1887. 1889. Toldt, C., Ueber das Wachsthum des Unterkiefers, Prager Zeitschr. f. Heilkunde, v., 1884. Wolff. J., Markirversuche am Scheitel, Stirn und Nasenbein der Kaninchen, Virchow's Archiv, Bd. 101, 1885; Ueber das Wachsthum des Unterkiefers, u. Beiträge zu den experimentellen Untersuchungen über das Knochenwachsthum, Virchow's Archiv, Bd. 104, 1888. Wolff, W., Ein Beitrag zur Lehre vom Knochenwachsthum, Verh. d. Berl. physiol. Gesellsch., Arch. f. Anat. u. Phys., Phys. Abth., 1884. Zachariadés, Recherches sur la structure de l'os normal, Comptes rendus de la société de biologie, 1889. |