TISSUES OF THE BODY. 245 



thickness from. O0045 to Q'0090 mm. In transverse sections they usually 

 lie in the middle of the lamellae, at times also between the same, with 

 their long axis parallel to the limiting edge of the lamella. General and 

 special lamellas display but little difference in this respect. The processes 

 of the lacunas, fine tubules of '00 14-0 '00 18 mm. in diameter, can only 

 be followed up for a short distance, when they disappear in the ground- 

 substance. 



But we gain a far more perfect insight into the arrangement of 

 these lacunas and canaliculi from sections of dried bone, in which the 

 former are filled with air, and are brought out with great sharpness, 

 appearing dark or black with transmitted and white with rejected 

 light, and constituting now the most striking form-elements in micro- 

 scopical investigation of the tissue, catching every eye (figs. 232, 

 233, 234, 237). From these jagged lacunte the canaliculi take their 

 rise in enormous numbers; traversing the ground-substance in an 

 irregular radiating course, and with many ramifications, and inosculat- 

 ing in great numbers with the processes of neighbouring lacunas. The 

 canaliculi likewise of one system of lamellae may pass over into another 

 adjacent to it. 



If we follow up these fine passages in a transverse section (fig. 238, ), 

 we see them in the first place converging 

 towards an Haversian canal, and opening into 

 the latter (&). Again, we can easily make out, 

 in the internal general lamellae bounding the 

 great medullary canal, the orifices of other 

 canaliculi, and also in the peripheral periostea! 

 leaves a third mode of exit externally under 

 the periosteum. 



In longitudinal section (fig. 232), we see 

 the lacunae surrounding the medullary canals 

 and some of their processes opening into the 

 latter in a more or less horizontal direction. 

 Those spots are specially instructive at which ^ v '' 



thp wall nf an 'RmiPrtinrt panal is pYnnQprl Fi &- 238. Lacunas (a, a) with their 

 01 an Uaverbian Canai IS exposed nume rous ramifications opening 



Which has been Opened longitudinally. Here tO a transversely-cut Haver$ian 



the numerous orifices of the canaliculi may be 



observed, giving to the surface a dotted appearance (fig. 232, e). The other 

 bones also show the same relations as those just described, with very many 

 modifications of course as to number and position. 



A glance at this so highly developed system of lacunas and canaliculi, 

 with its multitude of free openings, explains the fact that a thin section 

 of bone rapidly fills with air on drying, and on being subsequently 

 placed in oil or very liquid Canada balsam, that the air is again dis- 

 placed by the latter. It is an object well worthy of microscopical 

 examination this gradual expulsion of the air by the advancing oil. In 

 microscopical preparations put up in Canada balsam, we have not 

 unfrequently an opportunity of observing both conditions of the lacunae 

 and canaliculi. At one point we see the air retained, at another it has 

 been replaced by the Canada balsam. The whole may be injected 

 likewise with coloured liquids (Gferlach). 



The question as to whether the walls of this complicated system of 

 canals are formed of a substance differing from the remaining ground- 

 mass, or whether the system merely represents a series of lacunae 



