ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 41 



pact intercellular substance, which is also traversed by numerous 



canals, known as Haversian canals. These contain the nutritive 



blood-vessels and correspond exactly 



in their course and branchings 



to the latter. The intercellular 



substance consists of lamellse, which 



are arranged concentrically round 



the canals. The Haversian canals 



begin on the surface of the bone, 



which is covered by a vascular and 



nervous connective tissue layer, 



known as periosteum, and open 



into larger spaces (marrow spaces), 



which in the long bones occupy 



the axis of the bone, but in the FIG. 31a. Longitudinal section through a 



spongy bones have an irregular long bone (after KSlliker). e, Haver- 



OJ sian canal, 



distribution. 



In a seoond form of osseous tissue the cells themselves remain in 

 the outer part of the excreted intercellular substance, and only their 



*i K 



FIG. 314. Transverse section through a long 

 bone (after Kolliker). E, bone corpuscles ; 

 <?, Haversian canals ; L, lamellae. 



FIG. 31 c. K, spaces containing the "bone 

 corpuscles and their processes they 

 open into the Haversian canal, He 

 (after Kolliker), 



numerous processes, which run parallel to one another and are of 

 great length, are embedded in it. The intercellular substance, 

 which is hardened by the deposition of calcareous salts, is therefore 

 traversed by a great number of fine tubes. It is deposited on one 

 side only of jbhe cells, and in its origin recalls the hard carapace 

 of the Crustacea, which is similarly traversed by prolongations of 

 cells. 



This kind of osseous tissue, traversed by fine parallel tubes, is 



