THE PHASES OF LIFE. 857 



frame is the prominence of the lymphatic system. Not only are the lym- 

 phatic glands largely developed and more active (as is probably shown by 

 their tendency to disease in youth), but the quantity of lymph circulation 

 is greater than in later years. Characteristic of youth is the size of the 

 thymus body, which increases up to the second year, and may then remain 

 for a while stationary, but generally before puberty, has suffered a retro- 

 gressive metamorphosis, and frequently hardly a vestige of it remains behind. 

 The thyroid body is also relatively greater in the babe than in the adult ; 

 the spleen, on the other hand, which grows rapidly in early infancy, is not 

 only absolutely, but also relatively, greater in the adult. It need hardly 

 be said that the recuperative power of infancy and early youth is very 

 marked. 



807. It would be beyond the scope of this work to enter into the 

 psychical condition of the babe or the child, and our knowledge of the de- 

 tails of the working of the nervous system in infancy is too meagre to per- 

 mit of any profitable discussion. It is hardly of use to say that in the 

 young the whole nervous system is more irritable or more excitable than in 

 later years ; by which we probably to a great extent mean that it is less 

 rigid, less marked out into what, in preceding portions of this work, we 

 have spoken of as nervous mechanisms. It may be mentioned that stimula- 

 tion of the various cerebral areas, in newborn animals, does not give rise to 

 the usual localized movements. The sense of touch, both as regards pres- 

 sure and temperature, appears well developed in the infant, 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 Donders observed 

 normal binocular movements of the eyes in an infant less than an hour old. 

 The eye is (in man) from the outset fully sensitive to light, though of course 

 visual perceptions are imperfect. As regards hearing, on the other hand, 

 very little reaction follows upon sounds L e., auditory sensations seem to be 

 dull during the first few days of life ; this may be partly, at least, due to 

 absence of air from the tympanum and a tumid condition of the tympanic 

 mucous membrane. As the child grows up his senses rapidly culminate, 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 colors 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 vari- 

 ations 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. 



808. 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 dentition is completed 

 by the appearance of the second molars usually before the end of the second 

 year. 



809. Ahout 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 tem- 

 porary 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 



