330 THE TISSUES. 



They frequently present two distinct layers; the outer being dis- 

 tinctly g-ranular, the inner clear and transparent. The innermost 

 lamella of an Haversian rod is, however, sometimes transparent 

 throughout. The number of lamellae in each Haversian rod is 

 usually from 8 to 15 ; but sometimes there are only 4 or 5, and 

 occasionally as many as 18 to 22. The whole Haversian rod ave- 

 rages about T Jj of an inch in diameter. 



Between and in the substance of the lamellae, the lacunae and 

 pores exist. The lacunae have their long diameter curved so as to 



lie concentrically with the lamellae in a 

 transverse section of the Haversian rods, 

 their flat surface presenting towards the 

 Haversian canal. Their very numerous 

 pores produce a very close striation ra- 

 diating from the Haversian canals, as 

 T(\ shown by Fig. 218. The lacunae are 

 sometimes very numerous, at others very 

 f 1 scanty. In the former case, they are ge- 



/ nerally arranged in tolerably regular al- 

 '* ____ --"' ternation, or one behind another in the 



Transverse section of a part of the direction of the radius of the Haversian 



bone surrounding an Haversian ca- -i T-I -i > i 



nai ; showing the pores commencing rod. JB requently, however, they are very 

 at the inner surface, a, anastomo- irregularly crowded together, or are se- 



sing and passing from lacuna to m 



lacuna. Magnified about 300 dia- parated by wider interspaces. 



meters. (Tomes.) ' frQm 



aspect of the innermost lacunae penetrate into the Haversian canals. 

 From the edges and external aspect of the same lacuna other pores 

 are given off* which communicate with the proximate pores of the 

 more distant lacunae, and thus the Haversian rod is completely pe- 

 netrated by the pores and lacunae, and permeated by the nutritive 

 fluid contained in them. The various forms of lacunae and pores 

 are shown by Fig. 219. 



The outermost lamella is often of somewhat irregular outline 

 from its being the first formed in the pre-formed irregular Haver- 

 sian space. 



3. The interstitial osseous tissue between the Haversian rods, 

 when it exists in small quantity, frequently presents only one to 

 three lacunae in a transverse section (Figs. 220 and 217), both of a 

 rounded form and quite irregularly disposed. When more abund- 

 ant, it is distinctly lamellar, and the lacunas are more regularly dis- 



