CH. V.] 



BONE. 



61 



bone, e.g., the humerus or ulna, the arrangement shown in fig. 83 

 can be seen. 



The bone seems mapped out into small circular districts, at or 

 about the centre of each of which is a hole, around which is an 

 appearance as of concentric layers ; the lacunce and canaliculi 

 follow the same concentric plan of distribution around the small 

 hole in the centre, with 

 which indeed they com- 

 municate. 



On making a longitu- 

 dinal section, the central 

 holes are found to be 

 simply the cut extremi- 

 ties of small canals which 

 run lengthwise through 

 the bone, anastomosing 

 w i th each o ther by lateral 

 branches (fig. 84); these 

 canals are called Haver- 

 sian canals, after the 

 name of the physician, 

 Clopton Havers, who 

 fi rst accurately described 

 them. 



The Haversian 

 canals, the average 

 diameter of which is 

 -gifo 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 take up the lymph 

 exuded from the Haversian blood-vessels and convey it to the 

 substance of the bone which they traverse. 



The blood-vessels enter the Haversian canals both from with- 

 out, 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 dilatations. 



The lacunce are occupied by branched cells, which are called 

 bone-cells, or Itone-corpuselts (fig. 85), which very closely resemble 

 the ordinary branched connective-tissue corpuscles ; each of these 

 little masses of protoplasm ministers to the nutrition of the 



Fig. 84. Longitudinal section from the human ulna, 

 showing Haversian canal, lacunse, and canaliculi. 

 (Rollett.) 



