160 COGSWELL ON THE HUMAN TEETH. 



ing of approaching danger to the pulp and nerve, which is the life 

 of the tooth. 



The third portion is the enamel composed of solid prisms of 

 fibres about 15600ths of an inch in diameter, arranged side by side, 

 closely adherent to each other ; their length corresponds with the 

 thickness of the layer which they form ; these two surfaces present 

 the ends of the prism usually more or less hexagonal, and in 

 its perfect state contains an extremely minute quantity of animal 

 matter. 



The pulp cavity is in the centre of the tooth ; this contains the 

 nerves and blood vessels, branching off from the chamber of the 

 tooth, supplying each root or fang, and thus connecting through 

 the apex or apicial foramen, (almost as fine as the point of a fine 

 cambric needle,) with the facial nerve receive their supply, which give 

 the teeth life and sensation, and enables these organs when wound- 

 ed or diseased often times to talre on a healthy action, and forming 

 on which is frequently found secondary dentine, properly called osteo 

 dentine, or a new growth of tooth structure, as may sometimes be 

 found on the grinding surface of the superior or inferior central 

 incisors, where the articulation is such as to have caused the direct 

 contact of both upper and under, on the cutting edges, and by rapid 

 wearing away of the dentine, after the loss of the enamel, en- 

 dangering encroachment upon the pulp and nerve ; and for the 

 protection of this a wise provision, if sufficient time be given, is the 

 formation of this secondary dentine. 



This growth of osteo-dentine is well illustrated by Professor 

 Owen in speaking of the skull and teeth of the elephant, as follows : 



*' The musket-balls and other substances or foreign bodies which 

 are occasionally found in ivory, are immediately surrounded by osteo- 

 dentine in greater or less quantity. It has been a matter of wonder 

 often how such bodies should become completely imbedded in the 

 substance of the tusk, sometimes without any visible aperture, or 

 how leaden bullets may have become lodged in the solid centre of a 

 very large tusk without having been flattened. The explanation is 

 as follows : A musket ball, aimed at the head of an elephant, may 

 penetrate the thin bony socket and the thinner ivory parities of the 

 wide conical pulp-cavity, occupying the inserted base of the tusk ; 



