262 PHYSIOLOGY FOR DENTAL STUDENTS. 



jaw, the skin of the temple and external ear, the lower part of 

 the face and the lower lip. 



RELATIONSHIP OP THE FIFTH NERVE TO THE TEETH. In any in- 

 flammatory condition of the teeth, the terminations of the sen- 

 sory fibers become stimulated, causing extreme pain. This is 

 toothache. The relationship of the fifth nerve to the teeth ex- 

 plains why disturbance in the latter should often cause the pain 

 to be referred not to the tooth that is involved, but to some skin 

 area on the face. This is called referred pain. The skin areas 

 corresponding to the different teeth have been worked out by 

 Head, and are indicated in the accompanying diagrams (Figs. 

 48 and 49). Not only may the pain be referred to the skin area, 

 but this itself may become hypersensitive. There is, moreover, 

 in each area usually a maximal spot at which the pain and ten- 

 derness are most marked. 



The sensory nerve endings in the teeth are all of the nature of 

 pain receptors; there are no temperature or tactile receptors, 

 these latter sensations being particularly developed in the tongue 

 and lips (see p. 244). The pain receptors of the teeth, like those 

 of the cornea, react practically in full intensity to every strength 

 of stimulus. This explains why a small degree of irritation, as 

 that due to caries, may cause as painful a toothache as an in- 

 tense irritation. As we have already explained, the purpose of 

 painful or nocuous sensation is protective, causing, for example, 

 withdrawal of the irritated portion of the body or some move- 

 ment of offense (see p. 251). In the case of the teeth it serves as 

 a warning that something must be done to arrest whatever 

 condition is causing it. The enamel and cement are devoid of 

 nerve endings, which, however, are very abundant in the pulp, 

 and probably also in the dental tubules (Mummery). An inert, 

 sensationless exterior covering, a highly sensitive center, and 

 between these a moderately sensitive tissue, describes the sensi- 

 tiveness of a tooth. The sensitiveness of the pulp is so great as 

 to suggest that it is partly of the nature of a highly specialized 

 ncci-receptor, just as the taste buds and olfactory epithelium 

 are specialized receptors for taste and smell. The sensitiveness 

 of the teeth diminishes with advancing age. 



