HISTOLOGY OF THE HARD DENTAL TISSUES. 115 



the development of these several structures may tend to throw 

 some little light on the appearance of them in the mature state, as 

 presented to us in an ordinary section. 



The dentine of a tooth is exquisitely formed as an organ of 

 sensibility and of touch, like most papillary organs, and, therefore, 

 it needs to be covered by a substance which, while capable of 

 transmitting sharp, clear, and well-defined impressions, is itself 

 insensible. Such a substance we have in the enamel, whose histo- 

 logical characters we will first examine. 



If we look at a healthy, well-developed tooth we shall find not 

 only a good shape for the office it is designed to fulfil, but a 

 surface smooth, regular, and compact ; in texture it should be so 

 hard that you might strike a light with it when brought sharply in 

 contact with steel. 



There are two methods by which we may examine more closely 

 into the character of this structure. One is by sections cut longi- 

 tudinally to the axis of the tooth, and also transversely to the 

 course of the fibres ; a second plan is by decalcifying such sections 

 by placing them in a solution of one part hydrochloric acid to 12 

 parts of water when, especially in young and newly-developed 

 teeth, many interesting results may be obtained. 



If we take a tolerably thin section of a well-developed tooth, we 

 shall have no difficulty in demonstrating the enamel fibres or 

 prisms standing in a close and dense phalanx over that part of the 

 tooth which is seen projecting from the gum in young and healthy 

 subjects. It extends over that part in varying thickness, being 

 thickest at the apex, and gradually diminishing to the neck of the 

 tooth, which should be embraced by tbe gum. Its fibres should be 

 nearly uniform in width, and its structure tolerably clear and free 

 from granulations. The fibres do not run in one continuous course, 

 but are marked by transverse stria}, presenting somewhat the ap- 

 pearance of voluntaiw muscular fibre. They lie in close contact with 

 each other, without any demonstrable intervening substance, and 

 are placed at right angles to the surface of the dentine on which 

 they stand ; in this way the ends, and not their sides, are pre- 

 sented to any opposing substance, and thus great strength and 

 compactness are afforded by their union, for it is plain that, did the 

 sides of the prisms present themselves, and one or two became 

 detached, the whole structure would soon crumble down. In sec- 

 tions, taken transversely to the course of the fibres, it is said that 



