BLOOD-VESSELS OF THE TEETH. 259 



little more advanced the crown of the tooth is beginning 

 to wear away before the root is entirely completed. In 

 the adult the crown has lost its points and the root at- 

 tained its full development. Finally, in old persons, the 

 crown is sometimes completely worn away, whilst the root 

 still remains almost in a perfect state." The destruction 

 of the teeth is therefore regarded as a useful criterion in 

 determining the age. 



The individual varieties of teeth are numerous, and refer 

 to their number, situation, form, direction, and structure, 

 already treated under the head of irregularities of the teeth, 

 which see. 



The varieties according to race are extremely limited. 

 The blacks have them, it is considered, somewhat larger, 

 longer, and more oblique than the white. As to the cus- 

 tom of staining and filing, which some nations adopt, such 

 habits cannot properly be considered under the head of 

 varieties. 



SECTION IV. 

 BLOOD-VESSELS OF THE TEETH. (Fig. fS.) 



The arteries supplying the teeth come from the internal 

 maxiilcfry. This is a branch of the external carotid. The 

 external carotid is one of the two divisions of the common 

 carotid, and extends from the top of the larynx to the neck 

 of the condyle of the loiverjaw, and meatus auditorium externus. 



It is at the upper part of the larynx or thyroid cartilage 

 that the common carotid arteries make their division into 

 external and internal carotid. 



The common carotids (*opa, the head, <n>j, the ear,) arise 

 low down in the neck at its base. The right common ca- 

 rotid comes from the arteria innominata, the left from the 

 arch of the aorta. Both ascend the neck, covered first by 

 iheplatysma and superficial fascia, along the inner margin 

 of the sterno-cleido mastoid muscles, to the top of the thyroid 

 cartilage, where, as we have just stated, they divide into 

 the external and internal carotids. 



