908 THE SHAPES OF TEETH [ch. 



All ordinary teeth, as we have seen, have their own natural 

 curvature, less or more, which becomes more manifest and con- 

 spicuous the longer, they grow. We cannot suppose that the field 

 of force (internal and external) in which the narwhal's tusk develops 

 is so simple and uniform as to allow it to grow in perfect symmetry, 

 year after year, without the least bias or intention toward either 

 side ; we must rather suppose that the resistances which the growing 

 tusk encounters average out and cancel one another, and leave no 

 one-sided resultant. The long, straight, tapering tooth is commonly 

 said to have a "spiral twist," but there is no twist at all; the ivory 

 is straight-grained and uniform, through and through. The tusk, 

 in short, is a straight, right-handed, low-pitched screw or helix, with 

 several threads; which threads, in the form of alternate grooves 

 and ridges, wind evenly and continuously from one end of the tusk 

 to the other, even extending to its root, deep-set in the socket or 

 alveolus of the upper jaw. 



How this composite spiral thread is formed is quite unknown.^ 

 We have just seen that it is not due to any twisting of the dentinal 

 axis of the tooth. That it is uniform and unbroken from end to 

 end shews that the tooth somehow fashions it as a whole ; and that 

 it extends deep down within the alveolus is enough to shew that it 

 is not impressed or graven on the tooth by any external agency. 

 We note, as a minor feature, that the several grooves or ridges which 

 constitute the composite thread have their individual or accidental 

 differences; a broader or a narrower groove continues unchanged 

 and recognisable from one end of the tooth to the other; in other 

 words, whatever makes each ridge or groove goes on acting in the 

 selfsame way, as long as growth goes on. A screw is made, in 

 general, by compounding a translatory with a rotatory motion, and 

 by bringing the latter into relation with the mould or matrix by 

 which the thread is fashioned or imposed; and I cannot see how 

 to avoid believing that the narwhal's tooth must revolve in like 

 manner, very slowly on its longitudinal axis, all the while it grows — 

 however strange, anomalous and hard to imagine such a mode of 

 growth may be. We know that the tooth grows throughout life in 

 its longitudinal direction, the open root and "permanent pulp" 

 accounting for this ; and only by a simultaneous and equally con- 

 tinuous rotation (so far as I can see) can we account for the perfect 



