THE STRUCTURE OF THE ELEMENTARY TISSUES. 47 



from the Haversian blood-vessels, and conveying it still more intimately 

 to the very substance of the bone which they traverse. 



The blood-vessels enter the Haversian canals both from without, by 

 traversing the small holes which exist on the surface of all bones beneath 

 the periosteum, and from within by means of small channels which extend 

 from the medullary cavity, or from the cancellous tissue. The arteries 

 and veins usually occupy separate canals, and the veins, which are 

 the larger, often present, at irregular intervals, small pouch-like dilata- 

 tions. 



The lacunte are occupied by branched cells (bone-cells, or bone-cor- 

 puscles) (Fig. 54), which very closely resemble the ordinary branched 

 connective-tissue corpuscles; each of these little masses of protoplasm 



FIG. 53. FIG 54. 



FIG. 53. -Longitudinal section of human ulna, showing Haversian canal, lacunae, andcanaliculi. 



FIG. 54. Bone-corpuscles with their processes as seen in a thin section of human bone. (Rollett.) 



ministering to the nutrition of the bone immediately surrounding it, 

 and one lacunar corpuscle communicating with another, and with its 

 surrounding district, and with the blood-vessels of the Haversian canals, 

 by means of the minute streams of fluid nutrient matter which occupy 

 the canaliculi. 



It will be seen from the above description that bone is essentially 

 connective tissue impregnated with lime salts: it bears a very close 

 resemblance to what may be termed typical connective tissue such as the 

 substance of the cornea. The bone-corpuscles with their processes, 

 occupying the lacunae and canaliculi, correspond exactly to the cornea- 

 corpuscles lying in branched spaces; while the finely fibrillated structure 



