BOXE. 



47 



supplied also by a proper nutrient artery, which entering 

 at some part of the shaft so as to reach the medullary 

 canal, breaks up into branches for the supply of the marrow, 

 from which again small vessels are distributed to the inte- 

 rior of the bone. Other small blood-vessels pierce the arti- 

 cular extremities for the supply of the cancellous tissue. 



Notwithstanding the differences of arrangement just 

 mentioned, the structure of all bone is found, under the 

 microscope, to be essentially the same. Examined with a 

 rather high power, its substance is found occupied by a 

 multitude of little spaces, called lacuna, with very minute 

 canals or canaliculi, as they are termed, leading from them, 

 and anastomosing with similar little prolongations from 

 other lacunae (fig. 16). In very thin layers of bone, no 



Fig. 17.* 



other canals than these may be 

 visible; but on making a transverse 

 section of the compact tissue, e.g., 

 of a long bone, as the humerus or 

 ulna, the arrangement shewn in 

 fig. 1 6 can be seen. The bone seems 

 mapped out into small circular dis- 

 tricts, at or about the centre of each 

 of which is a hole, and around this 

 an appearance as of concentric 

 layers theZacimtzand canaliculi fol- 

 lowing the same concentric 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 (fig. 1 7), and 



* Fig. 17. Haversian canals, seen in a longitudinal section of the 

 compact tissue of the shaft of one of the long bones, a. Arterial canal ; 

 l>. Venous canal ; c. Dilatation of another venous canal. 



