The third stage is an action of a different nature, and is fulfilled in the absorption of the osseous matter interposed between the cells, and also of that portion of the ossified cells which lay in contact with the intercellular tissue of the columns.
By this change, the column, once composed of closed cells, is converted into a tube marked by numerous indentations corresponding in number to the cells which entered into its formation, there being a contraction at the points of junction between the cells. The tube so fortned, supposing this condition to be permanent, would have closed ends, and the length would be determined by the ends of the columns of cells from which it was formed. (See fig. 462.) These elongated tubes, I believe with the authors of " The Physiological Anatomy," to be the Haversian canals in their rudimentary state. They do not, however, retain this form of tubes with closed ends, but like individual cells become perforated and communicate with other tubes similarly formed ; but as these do not follow each other in straight lines, the openings are formed at the sides of the tubes instead of their ends, so that these commu nications are at angles with the tubes. These and other openings which are formed between the cells or tubes lying parallel to each other are those Haversian canals in their rudimentary state which traverse bone at right angles to its length, and form anastomoses with the longi tudinal Haversian canals, which in hone are by far the most numerous.
In the tubes no trace is left of the central nucleus or cavity of the original cartilage cor puscle; they contain, however, small spheri cal bodies composed of very minute granules. These are transparent and resemble in appear ance those peculiar globules found in the blood and cornmonly designated lymph globules. (See fig. 462).
They are very numerous, and, indeed, al most fill the cavity. They have a red tinge, and constitute almost entirely the mass of red matter found in the interior of all recently formed and forming bone. These bodies were described by Dr. Todd and AIr. Bowman, and they suppose them to be concerned in the developement of vessels, since up to this period of ossification no bloodvessels exist in the forming tissue, but make their appearance so soon as the tubes become pervious.
If a uansverse section be made of bone in this stage of its formation, there will be no difficulty in recognising, first, the ossified intercellular tissue, then the ossified parietes of the once cartilage corpuscle, which, being now the lining of the tube, will eventually be the external lamina of an Haversian systern ; and, lastly, the granular globules contained in the tubes.
The last point for consideration in the de velopement of bone is the formation of the hone-cells. Several re cent authors consider the cells to be formed from the nucleus of the cartilage corpuscle; I have not been able to confirm their state tnents, but have been led to entertain a diffe rent opinion of their origin. The formation of tubes having been completed, the inner layer formed from the parietes of the corpus cle is at first thin, and, could it be withdrawn entire, would look like a tube formed by the junction of a number of hollow beads. Partial separation may, however, be produced, as seen in fig. 463. This state of alternate dilatation and contraction seen in the tube at its first formation is soon lost, and the tube becomes of nearly equal diameter through out from the filling up of the dilatations by the deposition of osseous tnatter in the usual gra nular form. But in this filling up of the dila-i tations small cells are left, and these are the.
bone-cells in the rudimentary condition, and form the outer layer of cells in the Haversian system. (See lig. 463.) At first it is difficult. to distinguish the tubes, of the bone-cells, but if a section be taken. near the perfectedbone, they will be seen in va rious stages of dere lopement. At this late period of the formation,, bone-cells appear in the ossified intercellu lar tissue, which from • the first formed a con nection between thei systems of tubes, and which in fully formed . bone unites the Haver sian systems. With this we have bone in its ' perfect state.