COMPARATIVE ANATOMY. 



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 II. 



We never see more than one grinder, 

 and part of another, through the gum 

 in this oiumal. The anterior one is gra- 

 dually worn away by mastication : its 

 fangs and alveoli are then absorbed ; the 

 posterior tooth coming forwards 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 or denticuli, 

 and cuts the gum soon after birth. The 

 2d, which has eight or nine plates, has 

 completely 'appeared at the age of two 

 years. The 3d, formed of twelve or 

 thirteen, at six years. From the 4th to 

 the 8lh 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- 

 pleted 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 ja\v. 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 laminae 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 JEthi- 

 opicus. " Observations on the Structure 

 of the Teeth of Graminivorous Quadru- 

 peds ; particularly those of the Elephant 

 and sns ^Ethiopicus," Phil. Trans. 1799, 

 Part II. 



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 Structure 

 and Mode of Growth of the Wild Boar 

 and Animal Incognitum." Phil. Trans. 

 1801, Part II. 



It is remarkable, ih.-it the number of 

 cervical vertebrae 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 vertebrae vary much in 

 number ; the elephant has three, 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 ossa 

 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, having 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. 



BONES OF THE UPPKB EXTIIEMITT. 



We may assert as a general observa- 

 tion, that the four-component parts of the 

 upper extremities, viz. the shoulder, arm, 

 fore-arm, and hand, can be clearly shewn 

 to exist in the interior extremities of all 

 mammalia, however dissimilar they may 

 appear to each other on a superficial in- 

 spection, and however widely they may 



