STRUCTURE OF BONE. 



267 



deposits, and, in like manner, of having the parts first laid down removed by 

 subsequent absorption from within as well as from without. Even when the 

 full growth of the skeleton has been attained, nutritive changes still take place 

 in it ; and the continuance of these seems to be destined, not so much to supply 

 any waste occasioned by decomposition for this must be very trifling in a 

 tissue of such solidity as to keep the fabric in a condition in which it may 

 repair the injuries in its substance occasioned by accident or disease. The 

 degree of this reparative power we shall find to be proportional to the activity 

 of the normal changes which are continually taking place in the osseous tissue ] 

 and is thus much greater in youth than in middle life, and in the vigor of man- 

 hood than in old age. 



257. When the compact Osseous substance of the shaft of a long bone, or of 

 the superficial portions of a flat bone, is examined with the naked eye, it is seen 

 to possess a somewhat laminated texture ; the external and internal laminae of 

 the long bones being arranged concentrically round the medullary canal (Fig. 

 59, B, a), whilst in the flat bones they are parallel to the surface. Towards the 



Fig. 59. 



A. Transverse section of ulna, deprived of its earth by an acid. The openings of the Havorsian canals seen. 

 Natural size. A small portion is shaded, to indicate the part magnified in Fig. B. 



B. Part of the section A, magnified 20 diameters. The lines indicating the concentric lamelke are seen, and 

 among them the corpuscles or lacunae appear as little dark specks. 



extremities of the long bones, and between the external plates of the flat bones, 

 are a number of cancelli, or small hollows bounded by very thin plates of bone ; 

 these communicate with the medullary canal where it exists ; having, like it, an 

 extremely vascular lining membrane; and their cavities being filled with a 

 peculiar adipose matter. Even the hard substance of the bone is traversed by 

 canals, on which the name of "Haversian" has been bestowed, after their dis- 

 coverer ; these canals run for the most part in the direction of the laminse ; but 



