INTRODUCTION. XXV 



Striae, over the whole, or a part of the fibre ; and he conjectures that 

 if the enamel-fibre be a mass of the calcareous salts, surrounded by 

 an organic capsule, that the strise may then belong to the capsule and 

 not to the enamel-fibre. The later researches of Dr. Schwann add to 

 the probability of this conjecture, and the absence of the minute striee 

 in the enamel of fossil mammalian teeth, at least in the examples 

 which I have submitted to microscopic investigation, may depend 

 upon the destruction of the original organic constituent of the 

 enamel. 



The enamel-fibres are directed at nearly right angles to the sur- 

 face of the dentine, and their central or inner extremities rest in slight 

 but regular depressions on the periphery of the coronal dentine. Thus 

 in the human tooth, the fibres which constitute the masticating surface 

 are perpendicular or nearly so to that surface, while those at the lower 

 part of the crown are transverse, and consequently have a position best 

 adapted for resisting the pressure of the contiguous teeth, and for 

 meeting the direction in which external forces are most likely to im- 

 pinge upon the exposed crown of the tooth. The strength of the enamel 

 fibres is further increased by the graceful wavy curves in which they 

 are disposed ; these curves are in some places parallel, in others opposed ; 

 their concavities are commonly turned towards each other where the 

 shorter fibres, which do not reach the exterior of the enamel, abut by 

 their gradually attenuated peripheral extremities upon the longer fibres. 

 Other shorter enamel-fibres extend from the outer surface of the enamel 

 towards the dentine and are wedged into the interspaces of the longer 

 fibres. In the teeth of fishes, the calcigerous tubes or fibres of 

 the enamel, which ramify and subdivide like those of the dentine, 

 have their trunks turned in the opposite direction, or towards the 

 periphery of the tooth ; so likewise even in the human teeth the analo- 



