HARDWICKE'S SCIENCE-GOSSIP. 



217 



GRAPHIC MICROSCOPY. 



By E. T. DRAPER. 



No. XXII. — Transparent Section of Tooth of Ant-eater. 



FULL exposition of 

 the general and 

 comparative struc- 

 ture of teeth may 

 be found in standard 

 works of Physiology 

 and ordinary text- 

 books ; but in ex- 

 planation of the 

 singular diversity of 

 form and character 

 in a mammal tooth 

 as shown in the 

 plate, it is necessary 

 to describe generally 

 the typical con- 

 dition. 



The teeth of 

 vertebrates greatly 

 differ in character, 

 in the disposition of 

 the tissues, structure, position, numbers, and adap- 

 tability, not only for seizing and macerating food, 

 but as weapons of defence, attack, and instruments or 

 tools subservient to the economy, and habits of the 

 animal. They also denote age, sex, and are curiously 

 adapted to the quality and character of food ; to meet 

 these and many other exigencies, their structures and 

 condition necessarily divaricate into differences, to 

 such an extent as to be subservient to use and func- 

 tions ; the shape, character and organisation of a 

 tooth raises it to the importance of a zoological 

 touchstone and element in classification, reaching the 

 deepest researches of the palaeontologist ; every class 

 of the vertebrata — fossil or recent — may be distin- 

 guished by the moulding and contexture of this organ ; 

 notwithstanding that in the complex creatures of 

 past ages, strange approximations, and combinations 

 of class, exist ; the recognition of an extinct Batra- 

 chian, the Labyrinthodon, was determined by the 

 character of the convoluted folds of the dentine of its 

 tooth. 



No. 250.— October 1885. 



In the beautiful sections as now prepared for 

 microscopical investigation, these differences are so 

 admirably shown, as to raise them above "popu- 

 larity," and elegant as they may be as attractive 

 objects, to the student they are of deeper interest and 

 educational value. 



A typical tooth (human molar) may be said to 

 consist of three constituents ; in the centre a cavity 

 enclosing a soft dental pulp, freely supplied with 

 blood vessels and nerves ; surrounding this is the 

 dentine, the actual formed substance with radiating 

 canaliculi, covered on the surface of the exposed parts 

 by the enamel ; surrounding the imbedded portion 

 (the root or fangs) is a thin vascular structure, the 

 cement, also rich in blood-vessels. A section cut 

 horizontally would exhibit all these components ; 

 made transversely, the pulp cavity, the dentine, and 

 enamel only would be revealed ; such a preparation 

 discloses a typical condition of the parts, but without 

 deviating from the fundamental principle, varieties 

 and modifications are found in lamination, solidity, 

 contour, and distribution of the tissues. These 

 diversities are found in every class, and extended 

 modifications in species. 



It is common knowledge that the horns of some 

 animals, as the rhinoceros, are formed of a dense 

 compressed mass of hairs, and the component parts 

 of these compacted structures are easily distinguished 

 by the microscope. Such cohesions are found in 

 teeth, as seen in the plate. An infinite number, each 

 with its distinctive character, may be aggregated 

 into one mass ; to external appearance it is a single 

 tooth ; on microscopical examination it is found to be 

 a multitude locked together. This peculiar compound 

 intertexture is common in the class of fishes, but some- 

 what rare in the mammalia. The object depicted in 

 the plate is an instance of this peculiarity, and is 

 thus described by Professor Sir R. Owen. 



" Each tooth of the Cape Ant-eater (orycteropus) 

 presents a simple form, is deeply set in the jaw, but 

 without dividing into fangs ; its broad and flat base 

 is porous like the section of a common cane. The 



T 



