CHAP.V.] LACUNAE OF BONE. Ill 



with in the human subject; and some remarkable ones from the 

 lower animals are appended from the same source (fig. 24) . In the 

 true dental substance, which is a kind of bone, the lacunae are 

 almost entirely deficient, and the pores attain a very singular deve- 

 lopment, which will be described in a subsequent chapter. 



But though varieties are occasionally met with, yet, in the true 

 bone of man and the mammalia, the lacunae possess a very constant 

 form ; being somewhat oval, and more or less flattened on their 

 opposite surfaces. The two surfaces look respectively to and from 

 the nearest surface of the tissue, and meet in a thin edge. As pores 

 pass off equally from all parts of the lacunae, it follows that by far 

 the greater number pass to or from the surface of the bone; an ar- 

 rangement admirably adapted for the transmission of the nutritious 

 fluids. The pores passing from the edge principally serve to con- 

 nect together those lacunae that lie at nearly the same distance from 

 the surface. In fig. 22, the lacunae are seen on their surface ; in 

 fig. 23, on their upper edge. 



The lacunae have an average length of ^-gVo f an mcn > an ^ they 

 are usually about half as wide, and one third as thick. The dia- 

 meter of the pores is from 20 ^ 00 to j-^-oT f an inch. 



The osseous tissue, thus studded by thousands of flattened 

 lacunae, which lie for the most part in planes parallel to the surface, 

 has a decided disposition to split up into lamina, following the same 

 direction. This is more evident in the bones of old persons, and 

 may be generally promoted by maceration in dilute acid. It is 

 most apparent where the mass of material between two vascular 

 surfaces is great, and the series of lacunae numerous. It is pro- 

 bable that this lamellated structure depends in part on the mode of 

 development and growth of this tissue, and it perhaps contributes 

 to the perfection of the nutritive process within it. 



It will now be easy to comprehend the apparently complex 

 arrangement of the osseous tissue in the interior of bones. Let us 

 tiike, for example, one of the long bones. The entire vascular sur- 

 face consists of, 1, the outer surface, covered by the periosteum; 

 2, the inner surface, lined by the membrane of the medullary 

 cavity, and of the cancelli; 3, the Haver sian surface, or that 

 forming the canals of the compact tissue, and having in contact 

 with it the vascular network that occupies them, and which has 

 been already described. These involutions of the surface are so 

 arranged that no part of the osseous tissue is in general at a greater 

 distance than -p^- of an inch from the vessels that ramify upon them. 

 There is a layer of tissue on the exterior of the bone deriving its 



