196 



THE ORGANS 



lacunae of a Haversian system communicate with one another and 

 with their Haversian canal by means of the canaliculi. In Haversian 

 systems the fibres of the matrix (see page 105) run in some lamellae 

 parallel to the canal, in others concentrically. Adjacent fibres thus 

 frequently cross at right angles. 



(2) Interstitial (Intermediate or Ground) Lamella (Figs. 

 117 and 118).— These are irregular short lamellae, which occupy the 



spaces left between adjacent 

 Haversian systems. 



(3) Circumferential La- 

 mellae (Fig. 117). — These are 

 parallel lamellae which run in 

 the long axis of the bone, 

 just beneath the periosteum 

 and at the outer edge of the 

 central marrow cavity. Oc- 

 casionally circumferential 

 lamellae are absent, the Haver- 

 sian systems abutting directly 

 upon periosteum. 



Channels for the passage 

 of blood-vessels from the peri- 

 osteum to the Haversian 

 canals pierce the circumfer- 

 ential lamellae. They are 

 known as V olkmami' s canals, 

 and are not surrounded by 

 concentric lamellae as are the 

 Haversian canals, but are 



Fig. 202. — Transverse Section of Compact 

 Bone from Shaft of Humerus. X150 and 

 slightly reduced. (Sharpey.) (Technic i, p. 

 202.) Three Haversian canals with their con- 

 centric lamellffi and lacunae; canaliculi connect- 

 ing lacunae with each other and with Haversian 

 canal. Between the Haversian systems of 

 lamellae are seen the interstitial lamellae. 



mere channels through the 

 bone. Similar canals pass from the inner Haversian canals into the 

 marrow cavity. 



The Periosteum. — This is a fibrous connective-tissue membrane 

 w^hich covers the surfaces of bones except where they articulate, and 

 are covered wdth articular cartilage. It is firmly adherent to the 

 superficial layers of the bone and consists of two layers. The outer 

 layer is composed of coarse fibrillated fibres and contains the larger 

 blood-vessels. The inner layer consists of fine white fibres and 

 delicate elastic fibres which supix)rt the smaller blood-vessels. In 

 growing bone a third layer, the osteogcnetic layer, is present lying 



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