64 STRUCTURE OF TEETH. 



root divided into several fangs. In the upper jaw the first and 

 second molar teeth have three roots, sometimes four, which are 

 more or less widely separated from each other, two of the roots 

 being external, the other internal. In the lower there are but two 

 roots, which are anterior. and posterior; they are flattened from 

 behind forwards, and grooved so as to mark a tendency to division. 

 The third molars, or dentes sapientise, are smaller than the other 

 two ; they present three tubercles on the surface of the crown ; and 

 the root is single and grooved, appearing to be made up of four 

 or five fangs compressed together, or partially divided. In the 

 lower jaw the fangs are frequently separated to some distance from 

 each other, and much curved, so as to offer considerable resistance 

 in the operation of extraction.* 



Structure.^ The base of the crown of each tooth is hollowed into 

 a small cavity, which is continuous with a canal passing through the 

 middle of each fang. The cavity and canal, or canals, constitute 

 cavitas pulpae, and contain a soft and secreting vascular organ, 

 the pulp, which receives its supply of vessels and nerves through 

 the small opening at the apex of each root. 



The tooth is composed of three distinct structures ; the ivory or 

 tooth-bone, enamel, and a cortical substance or cementum. The 

 ivory consists of microscopic undulating and branching tubuli, 

 which open by their larger extremities upon the walls of the cavitas 

 pulpee and radiate towards the surface of the ivory, where they ter- 

 minate in ramifications of infinite minuteness. These tubuli have 

 distinct walls, are separated from each other by intervals equal in 

 breadth to the diameter of two or three tubes, and composed of 

 dense dental substance, and they contain within their cylinders a 

 calcareous substance disposed in irregular masses. J As the growth 

 of the tooth takes place from the surface towards the centre, the 

 most minute ramifications are first formed, and the trunks of the 

 tubuli are the last deposited. 



The enamel forms a crust over the whole exposed surface of the 

 crown of the tooth to the commencement of its root ; it is thickest 

 over the upper part of the crown, and becomes gradually thinner as 

 it approaches the neck. It is composed of minute hexagonal crys- 

 talline fibres, resting by one extremity against the surface of the 

 ivory, and constituting by the other the free surface of the crown. 

 The enamel is separated from the ivory by a thin layer of mem- 

 brane, continuous with a thin organic sheath which encloses each 

 enamel fibre, and marks it by means of transverse lines into irre- 

 gular divisions. Mr. Nasmyth is of 'opinion, that the enamel is in- 



* See a valuable little practical work, " On the Structure, Economy, and Pathology 

 of the Teeth," by Mr. Lintott. 



t The structure of the teeth was discovered by Purkinje and Retzius, and has been 

 farther prosecuted in this country by Mr. Nasmyth, to whose beautiful work, " Re- 

 searches on the Developement, Structure and Diseases of the Teeth," I must refer 

 those who may feel interested in this important subject. 



t The disintegrated condition of the calcareous substance is probably the effect of 

 desiccation ; it is very remarkable in decayed teeth. 



