THE STRUCTURE OF THE ELEMENTARY TISSUES. 69 



tween the 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 

 comparative lightness; they serve both to protect internal organs in the 

 trunk and skull, and as levers worked by muscles in the limbs; not- 

 withstanding their hardness they possess a considerable degree of elas- 

 ticity, which often saves them from fracture. 



The material of which the chief portion of the teeth is made up, 

 called Dentine, is frequently classed with bone and as one of the con- 

 nective tissues. The other constituents of the teeth also resemble bone 

 in structure to a considerable degree; it will be as well therefore to give 

 in this place some account of the teeth. 



The Teeth. 



During the course of his life, man, in common with most other 

 mammals, is provided with two sets of teeth ; the first set, called the 



Fig. 72.- Normal well-formed jaws, from which the alveolar plate has been in great part removed, 

 so as to expose the developing permanent teeth in their crypts in the jaws, i 



temporary or milk teeth, makes its appearance in infancy, and is in the 

 course of a few years shed and replaced by the second or permanent set. 



The temporary or milk teeth have only a very limited term of 

 existence. 



They are ten in number in each jaw, namely, on either side from the 

 middle line two incisors, one canine, and two deciduous molars, and are 

 replaced by ten permanent teeth. The number of permanent teeth in 



