STRUCTURE OF THE ELEMENTARY TISSUES. 



55 



fibrous and osteogenetic layers, it will never become enveloped in bone, 

 for all the bone is formed beneath the latter. 



Other varieties of connective tissue may become ossified, e.g., the 

 tendons in some birds. 



Functions of Bones. Bones form the framework of the body; for 

 this they are fitted by their hardness and solidity together with their com- 

 parative lightness; they serve both to protect internal organs in the trunk 

 and skull, and as levers worked by muscles 

 in the limbs; notwithstanding their hard- z- 



ness they possess a considerable degree of 

 elasticity, which often saves them from 

 fractures. 



TEETH. 



The principal part of a tooth, viz., den- 

 tine, is called by some a connective tissue, 

 and on this account the structure of the 

 teeth is considered here. 



A tooth is generally described as pos- 

 sessing a crown, neck, and fang or fangs. 



The crown js the portion which pro- 

 jects beyond the level of the gum. The 

 neck is that constricted portion just below 

 the crown which is embraced by the free 

 edges of the gum, and the fang includes all 

 below this. 



On making a longitudinal section 

 through the centre of a tooth (Figs. 58, 

 59), it is found to be principally composed 

 of a hard matter, dentine or ivory; while 

 in the centre this dentine is hollowed out 

 into a cavity resembling in general shape 

 the outline of the tooth, and called the 

 pulp cavity, from its containing a very 

 mass, composed of connective-tissue, blood-vessels, and nerves, which is 

 called the tooth-pulp. 



The blood-vessels and nerves enter the pulp through a small opening 

 at the extremity of the fang. 



Capping that part of the dentine which projects beyond the level of 

 the gum, is a layer of very hard calcareous matter, the enamel; while 

 sheathing the portion of dentine which is beneath the level of the gum, 

 is a layer of true bone, called the cement or crusta petrosa. 



FIG. 59. Premolar tooth of cat in situ. 

 Vertical section. 1 . Enamel with decus- 

 sating and parallel striae. 2. Dentine with 

 Schreger's lines. 3. Cement. 4. Perios- 

 teum of alveolus. 5. Inferior maxillary 

 bone showing canal for the inferior 

 dental nerve and vessels which appears 

 nearly circular in transverse section. 

 (Waldeyer.) 



vascular and sensitive little 



