STRUCTURE OF THE ELEMENTARY TISSUES. 



45 



On making a longitudinal section, the central holes are found to be 

 simply the cut extremities of small canals which run lengthwise through 

 the bone, anastomosing with each other by lateral branches (Fig. 4G), 

 and are called- Haversian canals, after the name of the physician, Cloptoa 

 Havers, who first accurately described them. The Haversian canals, the 

 average diameter of which is -^-^ of an inch, contain blood-vessels, and 

 by means of them blood is conveyed to all, even the densest parts of the 

 bone; the minute canaliculi and lacunae absorbing nutrient matter 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 



FIG. 46. 



FIG. 46. Longitudinal section of human ulna, showing Haversian canal, lacunae, and canaliculi. 

 FIG. 47. Bone corpuscles with their processes as seen hi a thin section of human bone. (Rollett.) 



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 

 dilatations. 



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

 puscles) (Fig. 47), which very closely resemble the ordinary branched 

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

 ministering to the nutrition ol the bone immediately surrounding it, 

 and one lacunar corpuscle communicating with another, and with its sur- 

 rounding district, and with the blood-vessels of the Haversian canals, by 



