268 



HARDWICKE'S SCIENCE-GOSSIP, 



This arrangement of the structures of the tooth 

 is modified in many of the lower animals ; in the 

 Rodent or gnawing animals, as the Rat, the enamel, 

 dentine, and cement are arranged in parallel layers ; 

 in order of hardness, the enamel, the hardest, being 

 in front, the dentine in the middle, and the softer 

 cement behind ; hence the tooth, as it wears away, 

 always presen'es a sharp chisel-like edge. In Herbi- 

 vorous Animals, as the Horse and Elephant, the 

 dentine, enamel, and cement in the molar teeth form 

 a series of parallel plates the unequal wear of which 

 preserves a rough surface for grinding. 



Teeth are formed, like hairs, from papillae of the 

 skin, or " mucous membrane " as it is termed when it 

 lines the cavities of the body. In the embryo, at an 

 early period, a groove is formed along the summit of 

 the gums ; this groove is then divided by transverse 

 folds into a number of separate cells at the bottom of 

 each of which is seated a papilla, the germ of the 

 future tooth. The edges of the cell then grow over 

 and convert them into shut sacs. The papilloe then 

 take the form of the future tooth, and the surface be- 

 comes calcified, forming the dentine, while the base 

 continues for a time to grow ; when, however, the 

 tooth has attained its full size the base of it closes 

 around the root of the papilla, preventing any further 

 increase in length, and leaving only a minute hole 

 through which the vessels and nerves of the tooth pass. 

 In certain cases, however, as the incisor teeth 

 of the Rodents, and the canine teeth or tusks 

 of the Elephant and other animals, this con- 

 traction of the base of the papilla does not 

 occur, so that the tooth continues to increase 

 in length for the whole of life, unless worn 

 away by use. The enamel is probably the 

 calcified epithelium of the inner surface of the 

 tooth sac. The second set of teeth are formed 

 in secondary sacs given off from the sacs of 

 the milk teeth. The molar teeth however 

 are exceptions, the first molar is formed in a 

 primary sac like the milk teeth, but the 

 second and third molars are fonned in second- 

 ary sacs, successively given off from that 

 of the first molar. Hence the first molar 

 of the] second set is developed like the teeth 

 of the first set, which will perhaps account 

 for the fact that it is usually the first tooth 

 to decay. 



In conclusion, I may remark that in no part 

 of the animal body do we find more obvious 

 and beautiful adaptations of structure to func- 

 tions than in the hard organs. 



The hard organs, being the most easily preserved 

 parts of the body, are of the greatest value to the 

 Zoologist and Palreontologist in determining the 

 structure, habits, and affinities of existing and extinct 

 animals. This value is due especially to the fact that 

 they constantly exhibit characters from which the 

 anatomy of the soft parts may be ascertained, as holes 



for the passage of vessels and nerves, rough marks 

 for the attachment of muscles and ligaments, &c. 

 From the shape of the bones of the limbs the mode 

 of progression may be ascertained, from that of the 

 teeth the nature of the food, and so on. To the 

 Paleontologist the hard parts are all important, for 

 they are almost the only data which he has for obtain- 

 ing any knowledge of the past inhabitants of our earth. 

 Thus, of the early representatives of orders of animals 

 composed wholly of soft tissues, as the Entozoa and 

 Tunicata, we know nothing ; whereas our knowledge 

 of the Corals, Echinodermata, Brachiopods, and 

 Reptiles, would be small compared to what it is, did 

 we not take into consideration fossil as well as existing 

 forms. 



NOTES ON TERATOLOGY. 



EXAMPLES of floral prolification, or monstrosi- 

 ties in the vegetable kingdom, are often met 

 with in one form or another ; probably lateral prolifica- 

 tion is of more common occurrence than the median 

 form. In one of the old stained-glass windows in the 

 Bodleian picture-gallery at Oxford is a representation 

 of a ranunculus, affected with median floral prolifica- 

 tion. It would appear from this that they have at- 

 tracted a little attention. 



Fig. 199. Cohesion of Leaflet in Clematis. 



Some forms are veiy interesting ; those figured 

 above are worthy of notice. We are indebted to the 

 kindness of our correspondents for the specimens. 

 We also give an example of the cohesion of the margin 

 of leaves. Dr. Masters gives a similar specimen 

 of pelargonium leaf. The one here figured is more 



