264 



BONE OR OSSEOUS TISSUE. 



lamellae. These were named " Haversian spaces " by Tomes and de Morgan, and 

 they are interpolated or intruded amongst the regular Haversian systems, some of 

 which may have been cut in upon in the excavation of the space. It was further 

 noticed by Tomes and de Morgan that the spaces in question may sometimes be seen 

 being filled up at one part by the deposition of lamellas, whilst they are extending 

 themselves by absorption at another. The Haversian spaces are most numerous in 

 young and growing bones, but they occur also after growth is completed. 



The three appearances above mentioned are due to the peculiar manner in which 

 the absorption of bone occurs ; for it is effected, as will presently be described, by 



Fig. 306. PORTION OP A NODULATED LAYER OF BONE-TISSUE FROM NEAR THE SURFACE OF THK 



SHAFT OF A DECALCIFIED HUMERUS (Sharpey). 



At one side shreds of fibrous lamellse are seen in the figure. Magnified 300 diameters. From a 

 drawing by Allen Thomson. 



the agency of large multinucleated cells, which excavate little hemispherical pits 

 (foveola3 of Howship) in the osseous tissue. If the process of absorption should 

 cease and should be succeeded by a re-deposition of osseous substance, the new 

 osseous matter filling up the hollows of the absorbed surface exhibits, when it is 

 detached, a raised impression corresponding with the hollows into which it fitted. 



In young bones the lamellar character is far less distinct than in adult bones, the 

 tissue being constituted chiefly of bundles of fibres which interlace in every direction 

 in the ground-substance ; in this reticular form of osseous tissue the lacunas are 

 both more numerous and irregular than in lamellated bone. 



When tendons become ossified, as is often the case, especially in birds, little but a 

 calcification of the ground-substance of the tendon occurs, so that, after decalci- 

 fying, the tendinous structure again becomes manifest. 



