Besides this relation between the tubes them selves and their cells, they have a very definite relation to the Haversian canals as well as to the free surface of the bone and also to tbe larninm.
The position occupied by the cells is between the lamince, or on the surface of the lamin ; and where concentric laminm occur, as in the Haversian system, the cells are arranged in cir cular lines between the laminm, each line of cells having as an exit common to it and the con necting laminm the Haversian canal. The flattened sides of the cells are parallel vvith the circumference of the Haversian canal, while their greater diameter is in the direction of the circular line of the lamina, or with the length of the canal to which they belonc,0-. Bone cells so placed send out numerous tubes, which pierce the lamince at right angles and proceed in great numbers to the vascular canal, into which they enter, there terminating in an open mouth upon the surface ; thereby establishing a con nection of tube channels between the bone cells of the Haversian system and the canal of the system. (Seefig.448.) Although these cells send out many tubes in the above direction, yet others, though comparatively few, take an opposite course, and then establish by anastomosis a con nection with the tubes of tbe surrounding bone cells. This is more particularly seen when we look upon a transverse section of an Haversian system ; but if a section taken in the length of an Ilaversian system be exarnined, the tubes will for the most part be seen dividing the cells equally in point of number from every part of the circumference of the cell, and of course pro ceeding in the length of the laminze between which the cell is placed. As in the transverse section of the liaversian system these tubes that take the longitudinal direction are not seen, so in this section the tubes proceeding directly towards the Itaversian canal are but badly shown. So many of the delicate tubes take the direction of the Ilaversian canals and enter it, that the parietes of each canal at first sight have a radiate appearance, which has led some writers to describe a system of radiate tubes passing through some of the laminm, but they have failed to trace their connection with the bone cells not far distant. When the
cell with its radiating system of tubes is situated near the surface of the bone, the direction of the latter will be mostly towards that surface, unless indeed there is a vascular canal near at hand, in which case many will proceed towards it.
Those cells which are placed in the connect ing laminT send out their tubes tolerably equally in each division, anastomosing freely with the tubes coming from the cells belonging to the Haversian or superficial system of laminm, and so establish a communication between the cells of the three systems of lamina.. The number of tubes and the size of the bone cells bear to each other no definite proportion ; thus a small cell may have many tubes while a much larg,er one has comparatively few. The number of the cells ln a given space is subject to con siderable variety, as well as the number of the radiating tubes, though generally the number of tubes will exist in inverse proportion to the num ber of the cells. Thus in the cmnia of small birds the cells are of very frequent occurrence, while the tubes connected with each cell are but few. Again, in dense bones of quadrupeds and of man, the cells are less frequent, but the tubes of each cell far more numerous.
Where the canals for vessels are very nu merous the bone cells become More rare, and in some cases tbey are nearly absent,as shown in fig. 451.
From the foregoing description it may be seen that the infinitely numerous tubes every where connected amongst the cells, converging at certain points and entering into cells, in fact form these cells ; that the cells are nothing more than many tubes coming to a point and losing their individual parietes.
In other cases where the tubes to each cell are not numerous, the cell itself may be com pared to a dilatation of those tubes. This view of the subject is bome out by the fact that even in the human subject we find here and there tubes occupying the place of the cells and their radiating tubes, while in certain fish the cells are almost entirely absent and the simple tubes general.