3°4 



PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



elephant is the most efficient fore-and-aft grinder imagi- 

 nable. The whole tooth, as seen (Fig. 189), is composed 

 of narrow transverse islands of dentine, surrounded by 

 ridges or plates of enamel separated by cement. The 

 section shows how this has been formed. Imagine a 

 mass of dentine from which spring many thin plates of 

 the same, each plate sheathed with enamel, and then 

 cement poured over the whole, and finally the tooth 

 subjected to wear. 



Mouth Armature of Whales. — Many cetaceans — 

 e. g., the sperm whales, porpoises, etc. — have teeth, but 

 these are conical, prehensile, not masticatory, teeth. 

 But the baleen (whalebone) whales have no teeth, but 

 in their stead have the most efficient food-taking ap- 

 paratus known. These animals have enormous heads 



A B 



Fig. 190.— Head of a whale : A, side view ; B, section. 



(nearly half the whole body), fifteen to twenty feet long 

 and ten to fifteen feet deep, and nearly the whole of this 

 great head is mouth. This huge cavern is largely occu- 

 pied with whalebone plates (Fig. 190). 



These horny plates, hundreds in number, are at- 

 tached above to the roof of the mouth, hang down, 

 and are split up into fibers at their edges, so that 

 the open mouth is like a moss-roofed cavern. The 

 hollow space beneath is filled up by the enormous 

 tongue. 



