44 CELL, DIFFERENTIATION AND THE ELEMENTARY TISSUES 



are called Haversian Canals, after the name of the physician, Clopton 

 Havers, who first accurately described them. 



The Haversian Canals. The average diameter of the Haversian canals 

 is 50 p.. They contain blood-vessels, and by means of them blood is con- 

 veyed to 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 from the 

 periosteum, and from within from the medullary cavity or from the can- 

 cellous tissue. The arteries and veins usually occupy separate canals. 



The lacuna are occupied by branched cells, the bone-cells or bone-corpus- 

 cles, figure 55, which very closely resemble the ordinary branched connective- 

 tissue corpuscles. The processes of the bone-cells extend into the canaliculi. 

 Each cell controls the nutrition of the bone immediately surrounding it. 

 Each lacunar corpuscle communicates with the others in its surrounding 



FIG. 55. Bone-Corpuscles with their Processes as Seen in a thin Section of Human Bone. 



(Rollett.) 



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

 ramifications just described. 



It will be seen from the above description that bone bears a very close 

 structural resemblance to what may be termed typical connective tissue. 

 The bone corpuscles with their processes occupying the lacunae and canalic- 

 uli correspond exactly to the cornea-corpuscles lying in the branched spaces. 



The Lamella of Compact Bone. In the shaft of a long bone three distinct 

 sets of lamellae can be clearly recognized: General or fundamental lamellae, 

 which are just beneath the periosteum and parallel with it, and around the 

 medullary cavity; Special or Haversian lamellae, which are concentrically 

 arranged around the Haversian canals to the number of six to eighteen around 

 each ; Interstitial lamellae, which connect the systems of Haversian lamellae, 



