





CHAP, in.] THE PHASES OF LIFE. 1149 



the human babe as regards the latter is intermediate between 

 the puppy and the young guinea-pig. As we have seen, the 

 fibres of the various tracts in the central nervous system ac- 

 quire their medulla at different epochs ; there is experimental 

 evidence in support of the view, otherwise probable, that the 

 assumption of functional activity follows in the same order ; 

 and the pyramidal tract is as we have seen the one in which 

 the fibres are very late in acquiring their medulla. It has 

 been asserted that in a new-born animal stimulation of the 

 vagus produces no cardiac inhibition and that this does not 

 appear for several days ; other observers however have ob- 

 tained 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 exercise, and in his early years he possesses a general 

 acuteness of sight, hearing, and touch, which frequently be- 

 comes 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. 



716. 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 

 corresponding 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 month, 

 the canines at about a year and a half, and the temporary den- 

 tition 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 per- 

 manent incisors replace their temporary representatives, fol- 



