BONE. 



1C 111 



manner of its disposition round the cavities being essentially the same in 

 both. 



Besides the openings of Haversian canals as above described, a trans- 

 verse section of the compact bone now and then presents vacuities or spaces 

 formed by absorption of the tissue. These are named "Haversian spaces" 

 by Tomes and De Morgan, who first showed that they occur not only in 

 growing bone but at all periods of life. In their primitive condition these 

 cavities are characterised by an irregular or jagged outline, and their forma- 

 tion by absorption is further indicated by their encroaching on the adjacent 

 groups of concentric lamellae, which have been, as it were, eaten away to 

 a greater or less extent to give place to the new cavity. In another stage 

 the spaces in question are lined by new formed lamellae, which may as yet 

 be confined to the peripheral part of the vacuity, or may fill it up in a 

 concentric series, leaving a Haversian aperture in the middle, and in fact 

 constituting a system of concentric Haversian lamellae, interpolated or 

 intruded among those previously existing. The concentric lamellae, which 

 thus come to occupy a greater or less extent of the area of the cavity, are 

 of course bounded exteriorly by segments of adjoining Haversian lamella), 

 which have been more or less cut in upon in the excavation of the space. 

 It has been further observed by Tomes and De Morgan, that vacuities may 

 sometimes be seen which are being filled up at one part by the deposition 

 of lamellae, whilst they are extending themselves by absorption at another. 

 The Haversian spaces are most numerous in young and growing bones ; but, 

 as already stated, they occur also after growth is completed. Their origin 

 and changes will be better understood after the reader has perused the 



Fig. XLIY. 



Fig. XLIV. TRANSVERSE SECTION OP COMPACT TISSUE (OP HUMERDS) MAGNIFIED ABPUT 



150 DIAMETERS. 



Three of the Haversian canals are seen, with their concentric rings ; also the corpuscles 

 or lacunas, with the canaliculi extending from them across the direction of the lamellae. 

 The Haversian apertures had become filled with debris in grinding down the section, and 

 therefore appear black in the figure, which represents the object as viewed with trans- 

 mitted light. 



