580 SPECIAL PHYSIOLOGY. 



only is a general, firm hold secured, but the notched edges of the laterally 

 compressed molars pass close by each other, like those of the blades of scissors. 

 In Insectivorous mammalia the motions of the jaws are almost equally limited. 

 In the Rodentia, besides shutting powerfully in gnawing, the jaw executes 

 rapid backward and forward movements, across the ridges of their molar teeth, 

 so as easily to grind tough vegetable substances. In the Herbivora, the lower 

 jaw is not limited to an up and down movement only, as in the Carnivora, 

 nor to that and a superadded backward and forward movement, as in the 

 Eodentia, but it is also capable of great lateral play. To permit this, the 

 glenoid fossa? are wide and shallow, the condyles of the lower jaw are short, 

 obtuse, and scarcely prominent, and the pterygoid muscles, which chiefly exe- 

 cute the lateral motions, are very large. The lower jaw is carried, during 

 mastication, in a sort of circular sweep, beneath the upper jaw, first forward 

 and to one side, and then backward and to the other, and so on, as may be 

 readily noticed in the cow, when chewing the cud ; the broad molar teeth, with 

 their unequal ridges and furrows of enamel and cement, are thus most effectu- 

 ally employed. In the edentulous ant-eater and ornithorhynchus, the condyles 

 of the jaws, and the glenoid fossse, are only slightly developed, and the move- 

 ments are comparatively simple and feeble. 



In Birds, the actions of the jaws are prehensile and not masticatory, ex- 

 cepting perhaps in the parrots, in which there is a lateral motion. For the 

 most part, the motion is hinge-like, and not very powerful, excepting in the 

 strong-billed birds, such as the finches, rooks, toucans, and Eaptores, which 

 latter hold and tear their food by movements of the head and neck. 



In Reptiles, whether they live on animal or vegetable food, the jaws are also 

 prehensile, or incisive, rather than masticatory, except in the herbivorous 

 iguanas, the molar teeth of which are large and tuberculated. In Amphibia, 

 the jaws are weak, and snapping or suctorial. 



In Fishes, the movements of the jaws are, likewise, for the most part, sim- 

 ply of a snapping and prehensile or incisive character ; but the pharyngeal 

 and other dentigerous bones within the mouth, are also movable, and these 

 bones, as well as the jaws, especially in the parrot- and wolf-fishes, are pro- 

 vided with strong masticating muscles, which make them act powerfully 

 against each other. 



Denticles of the Won- Vertebrate Animals. 



As already stated, true teeth are found only in the Yertebrata, but denticu- 

 lar organs are met with in some of the other Subkin^doms. Amongst the 

 Mollusca, the Cephalopods are provided with horny jaws, which open and 

 shut vertically. Some Gasteropods, also, have similar, but smaller, jaws 

 moving laterally ; but nearly all of them are provided with a peculiar strap- 

 shaped band, beset with rows of minute horny denticles situated in the mouth, 

 and often spoken of as the tongue. This organ, named by Huxley the odonto- 

 phore, is placed not on the floor, but on the roof, of the mouth. It is moved 

 backwards and forwards by appropriate muscles, and thus files or rasps very 

 hard substances ; as its anterior part is worn away, the odontophore, with its 

 horny denticles, is renewed within a special sac, seen at its hinder end. 



Amongst the Annulosa, the Arthropoda have mandibles, always found at 

 the sides of the mouth, and provided with strong muscles, which give them a 

 horizontal, not a vertical motion ; they are composed of calcareous or chitin- 

 ous substance, and differ remarkably in shape, in different species, being usu- 

 ally curved and pointed, and having serrated or dentated edges ; they are 

 strong in the actively feeding larvae, and also in most perfect Insects, as in 

 wasps and beetles. In the Crustacea, the mandibles are very strong, and in 

 certain species, as in the lobster, hard gastric tubercles, worked by powerful 

 muscles, exist at the entrance of the stomach. Even amongst the soft An- 

 nelids a denticular apparatus exists, as seen in the leech, which, by means of 

 three minute denticles, inflicts a tri-radiate wound. 



Amongst the Annuloida, many of the minute Rotifera possess complex den- 

 ticulated plates, which are worked transversely across the oral orifice. In 



