It does not appear that crusty petrosa is an essential part In the grinders of graininivorons animals. For those of the rhinoceros do not possess it, although the enantel descends into their substance, and forma cavity which is filled with the food, &e.
!home and !flake likewise state that it does not exist in the hippopotamus, where threre are internal productions of enamel! but Air, JIle.articy, the learned and In genious lecturer on comparative anatomy at St Ilartholoinew's Hospital, hits Novi it in small quantity on the exterior stir. liter, of the tooth near its root.
The want of satisfactory observations prevents its from saying nitwit on the change of the teeth, particularly in wild Some erroneous opinions of for. mer (knelt, as, for instance, that the do mesticated pig changes its teeth, and that 'the wild animal does not, hardly require an express coetradiction in the present lay, There is no animal of the class Marnmalia, where the first appeurance and subse quent removal of the deciduous teeth take place at so late a period of life AS in moo.
The permanent teeth are rnerally formed in cavities near the roots of the temporary ones; and they succeed to the vacancies left by the discharge of the hatter. A different mode of succession ol). tains, however, in some ;entailers. The adult molares of the human subject ore formed in the back of the two jaws, front which situation they advance suc cessively towards the front, in proportion us the maxillary hopes are lengthened in that direction. A similar, but much more remarkable, species of anecession is ob served in the grinders of' the elephimr, where it was ascertained by the labours of Mr. Corse, who has explained and illustrated the subject in a series of beau tiful engravings. See " Observations on the different species of Asiatic Elephants, and their Mode of Dentition," Phil. Trans. 1792, Part H.
We never see more than one grinder, and part of another, through the gum in this Alma'. The anterior one is gra dually worn away by mastication : its fangs and alveoli are then absorbed ; the posterior tooth coming fbrwards to sup ply its place. As this goes through the same stages as the preceding grinder, a third tooth, which was contained in the back of the jaw, appears through the gum, and advances, in proportion as the destruction and absorption of the other proceed. The same process is repeated at least eight times; and each new grin. der is larger than that which came before it. The first or milk grinder is compos ed of four transverse plates Dr denticuli, and cuts the gum soon after birth. The 2d, which has eight or nine plates, has completely at the age of two years. The 3d, formed of twelve or thirteen, at six years. From the 4th to the 8th grinder, the number of plates va ries from fifteen to twenty-three, which is the largest hitherto ascertained. The exact age at which each of these is com has not yet been made out. But it appears, that every new one takes at least a year more for its formation than its pedecessor.
From the gradual manner in which the tooth advances, it is manifest, that a small portion of it only can penetrate the gum at once. A grinder, consisting of twelve or fourteen plates, has two or three of these through the gum, whilst the others are embedded in the jaw. The formation of the tooth is complete, therefore, first at its anterior part, which is employed in mastication, while the back part is very incomplete ; as the succeeding lamina ad vance through the gum, their formation is successively perfected. But the pos terior layers of the tooth are not employ-. ed in mastication, until the anterior ones have been worn down to the very fang, which begins to be absorbed. One of these grinders can never, therefore, be procured in a perfect state : for if its an terior part has not been at all worn, the back is not completely formed, and the fangs in particular are wanting; while the structure of the back of the tooth is not completed, until the anterior portion has disappeared.
A similar kind of succession, but to a less extent, has been ascertained by Mr. Home, in the teeth of the sus lEthi opicus. "Observations on the Structure of the Teeth of Graminivorous Quadru peds ; particularly those of the Elephant and sus /Ethiopicus," Phil. Trans. 1799, Part H.
The researches of the same gentleman have also proved it to exist in the wild boar to a certain degree ; and have ren dered it probable that it occurred like wise in the animal incognitum (mam moth). " Observations on the Structurb and Mode of Growth of the Wild' Boar and Animal Incognitum." Phil. Trans. 1801, Part II.
It is remarkable, that the number of cervical vertebra in the mammalia should be constantly seven, although the animals of this class differ so much in the length of the neck. A single exception occurs in the three-toed sloth, which has nine.
The lumbar vertebra vary much in number ; the elephant has the cam el seven, the horse six, and the ass five. Mules have generally six. The os coc cygis is prolonged so as to form the tail of quadrupeds.
The cavity of the pelvis is so narrow in the mole, that it cannot hold the parts of generation, and the neighbouring viscera, which lie, therefore, externally to the ossit pubis.
In the kangaroo, and in other marsu pial animals, the anterior margin of the ossa pubis is furnished with a peculiar pair of small bones for supporting the abdominal pouch of the female.
Cetaceous animals, no hind feet, have consequently no pelvis : but there is a pair of small bones in the lower part of the belly, which may be compared to the ossa pubis.
In a very few mammalia, as some bats and armadillos, there is a pair of ribs less than in man ; but in most of the class these bones are more numerous. The horse has 18, the elephant 19, and the two-toed sloth 23 pairs. The sternum is generally cylindrical and jointed.