The anatomical and physiological differences between these two extreme types, e.g., the cat and the cow, are largely correlated with this fundamental contrast in food habits. The cat as a pro teid eater receives its food in a highly concentrated and elaborate form. Consequently its digestive tract is relatively short and simple, the stomach is not subdivided and the gastric juice shows a high percentage of hydrochloric acid.
On the other hand, the cow, representing the extreme carbo hydrate-feeding type, normally has in its stomach from 14 to 18% of its own total weight in bulky herbaceous food, which contains a great amount of cellulose. The cow's digestive juices are unable to dissolve this indigestible food, but the cow produces an enor mous quantity of saliva, which softens the food. By regurgitating and chewing the cud it mashes up the food in preparation for the fermentation caused by the anaerobic bacteria and infusoria in its stomach. In this way the mass gradually becomes fit for diges tion by the long digestive tract.
The differences in diet between the cat and the cow are reflected in the differences in their dental mechanism and associated parts of the skull, as set forth in the following table.
Relatively few mammals attain either of these extreme special izations, the majority subsisting on a more or less mixed diet.
Incisors and Canine Teeth.—The front teeth of mammals are adapted, according to the nature of the food, into insectivorous, carnivorous, gnawing, omnivorous, frugivorous, browsing, grazing, piscivorous and various derived and mixed types.
In the more primitive insectivorous forms the incisors (4 on each side in primitive placentals and 4 in primitive marsupials) are small nipping teeth, suitable for catching insects, the lower centrals slightly procumbent, the crowns simple with blunt points or slightly sharp edges. Tn the cat the incisors are fairly primitive in form but are nearly vertical, so giving a more powerful bite.
In rodents the incisors become long-crowned, curved, flattened cylinders with chisel-like edges growing continuously from per sistent pulps. In the lower Oligocene forerunners of the Probos cidea (q.v.) or elephants (Moeritherium), one pair of upper incisors and one pair of lowers are moderately enlarged and adapted for cutting and plucking vegetation from the ground. In Palaeomastodon the next higher stage, the upper incisor tusks are much enlarged ; through the development of a proboscis, they no longer oppose the lower incisors but function as levers and weapons. In certain later mastodons and elephants the lower
incisors are gradually eliminated and the upper incisors attain great size, reaching in the extinct Elephas ganesa to ro ft. 81 inches.
A reduction or loss of the incisors has frequently occurred in animals with a protrusile tongue, as in the sloths, anteaters, etc.
In the horse family, which are grazers, both the upper and lower incisors have become long-crowned, with cup-like insinkings from the incisal surface of the crown. This arrangement strength ens the tooth for cutting off siliceous grass stems, while the long crown insures a long period of use.
In the cow, representing the browsing and grazing ruminant artiodactyls, the upper incisors and canines have been replaced by a horny pad, the lower incisors and lower canines have spatulate crowns arranged in a semicircle, adapted, with the horny pad above, for plucking herbage.
The canine teeth of carnivores are peculiarly fitted for killing living prey. This type of tooth culminates in the great canines of the extinct sabre-tooth tigers. On the other hand, in herbivorous forms the canine teeth are either reduced or adapted to some other function, as in the lower canines of ruminants above men tioned, or the upper canines or fighting tusks of the boar.
Deciduous or Milk Teeth.—In mammals the true molars, although erupting late, appear to belong to the same series as the deciduous incisors, canines and premolars, but have long lost their permanent successors. Since as soon as the young animal is weaned it usually feeds upon the same food as its parents, it will be obvious that it usually needs a functional set of teeth of the same general kind as those of its parents but smaller. Thus the milk teeth are usually not notably different from the permanent set. The presence of the milk teeth makes it possible for the permanent teeth to grow to any desirable size before eruption, teeth once erupted no longer grow except at the roots. Usually the hindmost milk teeth are more molariform than the permanent teeth that replace them, apparently because the young animal needs such teeth where the bite is the strongest.