BONE. 



57 



various directions, but more or less curved (Fig. 27). These are termed interstitial 

 lamellae. Again, other lamellae for the most part found on the surface of the bone, 

 are arranged concentrically to the circumference of bone, constituting, as it were, 

 a single Haversian system* of the whole bone, of which the medullary cavity would 

 represent the Haversian canal. These latter lamellae are termed circumferential, 

 or by some authors primary or fundamental lamellae, to distinguish them from those 

 laid down around the axis of the Haversian canals, which are then termed secondary 

 or special lamellae. 



The Haversian canals, seen as round holes in a transverse section of bone at 

 or about the centre of each Haversian system, may be demonstrated to be true 

 canals if a longitudinal section is made, as in Fig. 29. It will then be seen that 

 these round holes are tubes cut across, which run parallel with the longitudinal 



FIG. 27. Transverse section of compact tissue of bone. Magnified about 150 diameters. (Sharpey.) 



axis of the bone for a short distance, and then branch and communicate. They 

 vary considerably in size, some being as large as T>^ of an inch in diameter ; the 

 average size being, however, about yl-g- of an inch. Near the medullary cavity 

 the canals are larger than those near the surface of the bone. Each canal, as a 

 rule, contains two blood-vessels, a small artery and vein ; the larger ones also con- 

 tain a small quantity of delicate connective tissue, with branched cells, the pro- 

 cesses of which communicate with the branched processes of certain bone-cells 

 in the substance of the bone. Those canals near the surface of the bone open 

 upon it by minute orifices, and those near the medullary cavity open in the 

 same way into this space, so that the whole of the bone is permeated by a system 

 of blood-vessels running through the bony canals in the centre of the Haversian 

 systems. 



The lamella? are thin plates of bone-tissue encircling the central canal, and 

 might be compared, for the sake of illustration, to a number of sheets of paper 

 pasted one over another around a central hollow cylinder. After macerating a 

 piece of bone in dilute mineral acid, these lamellae may be stripped off in a longi- 

 tudinal direction as thin films. If one of these is examined with a high power 

 under the microscope it will be found to be composed of a finely reticular struc- 

 ture, presenting the appearance of lattice-work made up of very slender, trans- 

 parent fibres, decussating obliquely, and coalescing at the points of intersection 

 so as to form an exceedingly delicate network. In many places the various 

 lamellae may be seen to be held together by tapering fibres, which run obliquely 

 through them, pinning or bolting them together. These fibres were first described 

 by Sharpey, and were named by him perforating fibres. 



