THE BONE. 1099 



canals if a longitudinal section is made, as in Fig. 628. It will then be seen that 

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

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

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

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

 the canals are larger than those near the surface of the bone. Each canal con- 

 tains two blood-vessels, with a small quantity of delicate connective tissue and 

 some nerve-filainents. In the larger ones there are also lymphatic spaces and 

 branched cells, the processes of which communicate, through the canaliculi, with 

 the branched processes of certain bone-cells in the substance of the bone. Those 

 canals near thp 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 lamelloi are thin plates of bone-tissue encircling the central canal, and 

 may 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 lon- 



FIG. 626 Transverse section of compact tissue of bone. Magnified about 150 diameters. (Sharpey.) 



gitudinal 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 struct- 

 ure, 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. These fibres are composed of fine 

 fibrils, identical with those of white connective tissue. The intercellular matrix 

 between the fibres has been replaced by calcareous deposit, which the acid dissolves. 

 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 (Fig. 630). 



The lacunce are situated between the lamellae, and consist of a number ol obi 

 spaces. In an ordinary microscopic section, viewed by transmitted light, they 

 appear as dark, oblong, opaque spots, and were formerly believed to be solid cells. 

 Subsequently, when it was seen that the Haversian canals were channels whicl 

 lodge the vessels of the part, and the canaliculi, minute tubes by which the plasma 

 of the blood circulates through the tissue, the theory was formulated that the 

 lacunae were hollow spaces filled during life with the same fluid, and only lined 



