408 DENTITION. [BOOK iv. 



very late in acquiring their medulla. It has been asserted that in a 

 iH'w-lmni animal stimulation of the vagus produces no cardiac 

 inhibition and that this does not appear for several days ; other 

 observers however have obtained positive results and that even in 

 the uterus ; probably in this respect also animals differ. In the 

 human infant the sense of touch, both as regards pressure and 

 temperature, appears well developed, as does also the sense of 

 taste, and possibly, though this is disputed, that of smell. The 

 pupil (larger in the infant than in the man) acts fully, and normal 

 binocular movements of the eyes have been observed in an infant 

 less than an hour old. The eye is from the outset fully sensitive 

 to light, though of course visual perceptions are imperfect. 

 Auditory sensations on the other hand, seem to be dull, though 

 not wholly absent, during the first few days of life ; this may be 

 partly at least due to absence of air from the tympanum and to a 

 tumid condition of the tympanic mucous membrane. As the child 

 grows up his senses sharpen with constant exejfcise, and in his early 

 years he possesses a general acuteness of sight, hearing, and touch, 

 which frequently becomes blunted as his psychical life becomes 

 fuller. Children however are said to be less apt at distinguishing 

 colours than in sighting objects ; but it does not appear whether 

 this arises from a want of perceptive discrimination or from their 

 being actually less sensitive to variations in hue. A characteristic 

 of the nervous system in childhood, the result probably of the more 

 active metabolism of the body, is the necessity for long or frequent 

 and deep slumber. 



976. Dentition marks the first epoch of the new life. At 

 about seven months the two central incisors of the lower jaw make 

 their way through the gum, followed immediately by the corre- 

 sponding teeth in the upper jaw. The lateral incisors, first of the 

 lower and then of the upper jaw, appear at about the ninth month, 

 the first molars at about the twelfth mouth, the canines at about a 

 year and a half, and the temporary dentition is completed by the 

 appearance of the second molars usually before the end of the 

 second year. 



About the sixth year the permanent dentition commences by 

 the appearance of the first permanent molar beyond the second 

 temporary molar ; in the seventh year the central permanent 

 incisors replace their temporary representatives, followed in the 

 next year by the lateral incisors. In the ninth year the temporary 

 first molars are replaced by the first bicuspids, and in the tenth 

 year the second temporary molars are similarly replaced by the 

 second bicuspids. The canines are exchanged about the eleventh 

 or twelfth year, and the second permanent molars are cut about 

 the twelfth or thirteenth year. There is then a long pause, 

 the third or wisdom tooth not making its appearance till the 

 seventeenth, or even twenty-fifth year, or in some cases not 

 appearing at all. 



