238 THE DIGESTIVE ORGANS. 



vessels are richly supplied with vasomotor nerves, the neuraxes 

 of sympathetic neurones, which terminate on the muscle-cells of 

 the vessels. In the adventitia are also found free sensory nerve- 

 endings. (See Fig. 177.) 



\. THE TEETH. 



The human dentition comprises twenty temporary or milk teeth, 

 namely, above and below, four incisors, two canines, and four 

 molars, which are replaced by thirty-two permanent teeth, con- 

 sisting of four incisors, two canines, four premolars, and six molars 

 for each jaw. Each tooth consists of a crown, which projects above 

 the gums, a relatively short and narrowed portion known as the 

 neck, and a portion which fits accurately into the alveolus and 

 is known as the root. For the variations in shape which the 

 different kinds of teeth present, the reader is referred to the text- 

 books of anatomy or to special works dealing with this subject. 



Structure of the Adult Tooth. The adult tooth is made up 

 of three substances the enamel, the dentin, and the cementum. The 

 latter covers that part of the tooth within the alveolar process of 

 the jaw and also the root of the tooth. The enamel caps that part 

 of the tooth projecting into the oral cavity, the crown of the tooth. 

 The neck of the tooth is the region where the cementum and 

 enamel come in contact. The greater part of the tooth consists of 

 dentin, which is present in the crown as well as in the root. All 

 the substances of the tooth just mentioned become very hard from 

 the deposition of lime-salts. Every tooth contains a cavity sur- 

 rounded by dentin, the pulp cavity, or dental cavity. This is filled 

 with a soft tissue, the pulp, consisting of white fibrous tissue, ves- 

 sels, and nerves. That part of the pulp cavity lying in the axis of 

 the fang is called the root-canal ; by an opening in the latter (fora- 

 men apicis dentis) the pulp is connected with the periosteal con- 

 nective tissue of the dental alveolus. 



The enamel is a very hard substance, the hardest in the body, 

 and may be compared to quartz. In uninjured teeth the enamel is 

 covered by an exceedingly thin, structureless membrane, the cuti- 

 cula dentis or Nasmyth's membrane, which varies in thickness, 

 measuring from 0.9 // to 1.8 //. It is very resistant to acids and 

 alkalies. On its under surface it often shows small pits, into which 

 project the ends of the enamel prisms. The enamel contains very 

 little organic substance (from 3 <f to 5^), "in consequence of which 

 it is soluble in acids with scarcely any residue. The elements 

 composing it are prismatic columns, the enamel prisms, which 

 probably occupy the whole thickness of the enamel from the 

 superficial membrane to the dentin. They are slightly thicker at 

 the surface of the tooth than at the dentin, and in transverse 

 section show a hexagonal or polygonal shape, and measure 

 from 3 fj. to 6 JLL in diameter. They often show quite regular 



