Chap. V.] GENERAL HISTOLOGY. 71 



the compact bone above described. This is called spongy-bone. 

 Many of the Haversian canals have been hollowed out into 

 large Haversian spaces, which may show a tendency to run one 

 into another. 



It is clear that the ossified matter of the bone-lamellae is an 

 intercellular substance a substance produced by the bone-cells 

 answering to that which is transparent and hyaline in cartilage, 

 and fibrillated in fibrous connective tissue. In a bone softened 

 with hydrochloric acid some of the lamellae may be torn off in 

 shreds. The matrix thus obtained shows crossing fibres ; and 

 from the lamella processes are given off which project at right 

 angles to the lamella and pass through holes in adjacent lamellae. 

 The holes for the admission of such processes will also be 

 seen. 



The bone* of the pigeon is essentially similar to that of the 

 rabbit. But a cross-section of the bone of a cod-fish shows a 

 structure very different from that seen either in the frog or in the 

 rabbit. It is more like ossified or calcified cartilage, there being 

 none of the lacunas or lamellae so characteristic of true bone. 



6. Teeth. Tooth sections may be prepared either from the 

 dry tooth by grinding, or from the decalcified tooth by the 

 ordinary process of section-cutting. 



In the interior of the tooth, passing up from the bottom of 

 the fang, is the pulp-cavity, empty in the dry tooth, filled with 

 vascular and nervous pulp in the living state. Around this, 

 forming the mass of the tooth, is the dentine, through which 

 there run a number of slightly waving or spiral tululi, sometimes 

 giving off fine branches. In the fresh state each contains a 

 fibre connected with the cells (odontoblasts) which line the pulp- 

 cavity. At their outer ends the tubules pass into a network 

 of intercommunicating spaces. Thus the dentine is an inter- 

 cellular substance formed between the fibres of, and due to the 

 vital activity of, the odontoblasts. 



The material encrusting the dentine differs according as our 

 section passes through the crown or the fang. In the crown it 

 consists of enamel (Fig. 25, vi.), composed of enamel prisms, set 



