The Haversian canals undoubtedly give pas sage to bloodvessels, which is their principal, if not their only purpose. Whether they con tain one or more vessels seems to admit of a little eloubt. Dr. Carpenter, in his work on Physiology, states that they contain an artery and vein. From my own observation I am not able to confirm his view. Indeed I am disposed to the opinion that they give passage to one vessel only ; that the larger canals which are found entering the bone convey an artery; that it divides from time to time after the manner of the canals described ; and that the vessels emerge again from the surface of the bone as capillaries. This branch of the subject requires some further investigation. The fore going observations apply only to dense bone. Where bone is cancellated for the reception of fat, the vessels occupy but a small space in the cancelli.
Of the corpuscles or cells of bone ; also called lucurue by Dr. Todd and Mr. Bowman.— The so-called corpuscles are nothing more than small cells existing in the substance of the tissue, and might with propriety be called bone cells. Some anatomists have designated them The size of the Haversian canals takes a con siderable range, varying from the uhrd to the 3iDth of an inch, as stated by Mr. Smee. In the young subject they seem larger than in the old. But by far the most marked difference in size of these canals is to be observed in the antlers of the stag at different periods of their growth. At an early period of the existence of the antler, the vascular canals are large and numerous, while at the time of their completion in size the canals are less numerous in an equal space, and very small: indeed many seem all but obliterated. The density of bone is pro duced more by the small size of the canals than by their comparative infrequency, though un doubtedly they are less frequent in the compact bone, as that composing the shafts of long bones.
In tracing individual canals, it will be found that the majority maintain the same size as far as we can follow them. This is not, however, observable in all. If a large canal be taken where it first enters the substance of the bone, it may be found giving off branches from time to time in various directions, and then again sending off smaller branches, which anastomose freely with each other, often joining at right angles.
Although it is very easy to trace a large canal pervading a bone, and then dividing from time to time into smaller ones, I have never been able to satisfy myself that these small canals again unite to form a second large canal, and thus to leave the bone. I am therefore led to the opinion that such does not occur, but that the small even-sized canals open and calcigerous cells, from the supposition that they contain in their interior an amorphous salt of lime. That this view is incorrect will
be subsequently shown. The cells cannot be described as having any definite unvarying shape or size. The general form is a com pressed oval, though not unfrequently they are circular, but flattened from side to side. Arpin, they are sometimes almost triangular in their outline, while in other instances they approach a linear shape. These are the most common va rieties of outline to which the bone cells are sub ject ; as they occur in the bones of man and the higher animals. But connected with the cells are numerous delicate branching tubes, which are slightly dilated as they enter the cells. The number arising from each cell does not allow if any very definite enumeration since no two ells will be found possessed of a'like number of iiranching tubes. The. general arrangement of the Obes is radiate as regards the cells, which forms heir common centre. This statement requires tome qualification for not uncommonly a much treater number oitubes arise from one side of lie cell than from the other, and these tubes all take one direction. A tube after passing some little distance from the cell will in many in stances divide, and each division pass on distinct from its fellow, equalling in size the parent tube. Frequent anastomoses are effected between different tubes arising from the same cells but far more frequently between those which arise from neighbouring cells. So fre quent are the connections that a free com munication is established between the various cells and branching tubes throughout the substance of the bone.
So numerous are the connections between the tubes, and immediately between the cells through the tubes, that a fluid introduced into one cell in a bone may find its way into every other cell of the bone. Indeed this does take place, though not from a single cell, yet from the surface of the bone. If, for instance, you place a bone that is dry, and opaque as a conse quence of being dry, in spirits of turpentine, in a very little time this bone, before opaque, will become comparatively transparent, and this through the fluid having passed through the tubes into the cells. For, as will be shown, it is the tubes only that open upon the surface of the bone, either the external surface or the sur face of the canals for vessels. Indeed, if a thin section of bone be taken and all moisture removed, and spirit of turpentine be added to it, when under the microscope, the passage of the fluid through the tubes may be seen, an ex periment suggested by my friend Mr. Bowrnan.