76 HISTOLOGY. 



3. Outer ground lamellae form the outer layers of the bone, 

 and are situated directly under the periosteum. 



4. Inner ground lamella form the boundaries of the med- 

 ullary cavity and are arranged concentrically around it. 



The outer ground lamellae are in places pierced by canals 

 which carry blood-vessels from the periosteum. These are 

 known as VoUcmann's canals. 



All these systems of lamellae are joined with one another 

 by a cement substance. If this is abundant, it forms the so- 

 called cement lines of v. Ebner, which separate the adjacent 

 systems (Figs. 41 and 42). 



The structure of the intercellular substance (ground sub- 

 stance) is fibrillar. These fibrils, which are capable of produc- 

 ing gelatin, are joined into bundles by means of homogeneous 

 interfibrillar cement substance. The bundles are joined in 

 turn by interfascicular cement substance. The fibril bundles 

 run parallel to one another and make up the lamellae. These 

 are often so arranged that the bundles of adjacent lamellae lie 

 at right angles to one another. An example of this is shown 

 in a cross-section of the compact substance of a long bone 

 (Fig. 41). The longitudinal fibril bundles are cut transversely, 

 while those in the adjacent lamellae running concentrically are 

 cut longitudinally. On examination with polarized light it is 

 found that the bundles which are cut longitudinally are doubly 

 refractive, while those that run concentrically around the 

 Haversiari canal are singly refractive. Thus in the crossing 

 of Nicol's prisms the former appear black and the latter light 

 (Figs. 41 and 42). 



This lamellated intercellular substance is found in all adult 

 bones. The ground substance with coarser fibres is found 

 mainly in embryos or only in special places in adults (e. g., in 

 the points of insertion of tendons). 



In the intercellular substance we find bundles of connective- 

 tissue fibrils which are quite independent of the lamellar fibrils. 

 They pass through the lamellae from the periosteum trans- 

 versely or diagonally (Fig. 41). These are known as Sharpey't 

 fibres, and remain only partially or not at all calcified. They 



