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HANDBOOK OF PHYSIOLOGY. 



the compact tissue as of a long bone, e.g., the humerus or ulna, the 

 arrangement shown in fig. 60 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 lacuna and canaliculi following the same con- 

 centric plan of distribution around the small hole in the centre, with 

 which indeed they communicate. 



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. 61)., 



Fig. 61. Longitudinal section from the human ulna, showing Haversian canal, lacunas, and 



canaliculi. (Rollett.) 



and are called Haversian canals, after the name of the physician, Clopton 

 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 con- 

 veying 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 be- 

 neath 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. 



