i. 3, the third incisor; c, the canine; p. 1, p. 2, p. 3, the premolars; m, the molars.
In the lower jaw, the grinding series is represented by only three teeth, all more or less resembling the second of the series in the upper jaw. Of these the third is the largest, and is called the lower carnassial, biting, as it does, against the upper tooth of that name. In every case the teeth of the lower jaw bite within those of the upper, and, the jaws being so articulated as to allow only of up and down motion, and being incapable of play from side to side, the molars and premolars entirely lose their character of grinders, and become trenchant, cutting up the food, in fact, in precisely the same manner as a pair of scissors.
Now comes the question, which of these teeth are premolars, and which molars? This is decided by finding which of them have their place occupied in the young kitten by its first set of back-teeth, the deciduous or milk molars, and which, on the other hand, have no predecessors: those which replace the milk molars being the premolars of the adult, those which arise as altogether new teeth, and have no representatives in the young animal, molars. The examination of a young Cat shows that there are, behind the canines, in the upper jaw three, and in the lower two teeth; that is to say, one less on each side of each jaw than in the adult. As age advances these deciduous or milk molars all drop out, and are replaced by the permanent premolars, while behind the last milk molar of each jaw an entirely new tooth makes its appearance—the true or permanent molar. Thus it is seen that only the last tooth in each jaw is a molar, and that the carnassials are of different natures in the two jaws, the upper being the last (third) premolar, the lower the single molar.
MILK TEETH OF LION, EXPOSED BY CUTTING AWAY THE OUTER PORTIONS OF BOTH JAWS.
(Natural size. From Owen, after Rousseau.)
d.i, deciduous incisors; d.c, deciduous canines; d.m.1, d.m.2, d.m.3, deciduous molars. The remaining letters have the same significance as in the [preceding figure].
We therefore write the premolars of the Cat (3–3)(2–2), and the molars (1–1)(1–1), so that the whole “dental formula” is as follows:—i., (3–3)(3–3), c., (1–1)(1–1), p., (3–3)(2–2), m., (1–1)(1–1) = 30. In the milk dentition, the number of incisors and canines is the same as in the adult, and, as we have just stated, the molars are absent, so that the formula is di., (3–3)(3–3), dc., (1–1)(1–1), dm., (3–3)(2–2) = 26, di, dc, dm, standing for deciduous incisors, canines, and molars.
The tongue in this family becomes an important adjunct to the teeth, almost losing its character as a delicate organ of taste. The little elevations or papillæ which beset the tongue in all animals—in ourselves for instance—are formed into strong horny spines set closely together like the teeth of a file, and, as may be seen any day at feeding-time at the Zoological Gardens, used to rasp the flesh from the bones as effectively as any file would do it. Most people must have noticed the different texture of a Cat’s and a Dog’s tongue. In the latter it is as smooth as in ourselves, in the former it has more of the texture of a piece of coarse sandpaper.
In some Felidæ, such as the Domestic Cats, the pupil, or small aperture in front of the eye which lets in light to the sensitive retina beyond, has the round shape it possesses in man, only in the dark, when it is dilated to receive every ray of light available. In the day, on the other hand, when more light is to be had than the animal requires, the pupil contracts to an ellipse, or in the strongest light to a mere line. This is not the case in the larger Cats, such as the Lion, Tiger, and Leopard, in which also the eyes themselves and the cavities in the skull for their reception are smaller, proportionally, than in the Domestic Cat.
Taking the structure of the Cat tribe, all in all, there is nothing whatever to make it the least difficult to suppose that they all sprang from one stock, and that size and colour, and every other point in which they now differ from each other, may have been brought about, through long periods of time, as the result of the influence of their surroundings. It is necessary to presume this, for classifiers from necessity lay hold on the most minute differences, for the sake of making proper specific distinctions, although these differences may be merely the outcome of some change of locality, warmer, or colder, drier, or moister, higher upon the hills, or lower down on the plains. Once developed, however, it becomes hereditary, and then a variety becomes a race, and a race solidifies into a species. Yet, the result once obtained, however it arose, the profit is great to us who are careful observers and enthusiastic admirers of the infinite fecundity of Nature.