XX INTKODUCflOtf. 



the horse as presenting the form of angular needles, 

 about -g-fiVoth f an i QC h m diameter, which are trav- 

 ersed by minute and close-set transverse striae over 

 the whole or a part of the fiber; and he conjectures 

 that if the enamel-fiber be a mass of the calcareous 

 salts, surrounded by an organic capsule, that the striae 

 may then belong to the capsule, and not to the enamel- 

 fiber. The later researches of Dr. Schwann add to the 

 probability of this conjecture ; and the absence of the 

 ininute striae in the enamel of fossil mammalian teeth, 

 at least in the examples which I have submitted to 

 microscopic investigation, may depend upon the de- 

 struction of the original organic constituent of the 

 enamel. 



"The enamel-fibers are directed at nearly right 

 angles to the surface of the dentine, and their central 

 or inner extremities rest in slight but regular depres- 

 sions on the periphery of the coronal dentine. Thus 

 in the human tooth, the fibers 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 impinge upon the exposed 

 crown of the tooth. The strength of the enamel-fibers 

 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 toward each other, where the 

 shorter fibers, which do not reach the exterior of the 

 enamel, abut by their gradually attenuated peripheral 

 extremities upon the longer fibers. Other shorter fibers 

 extend from the outer surface of the enamel toward 



