DEVELOPMENT OF THE BODY AFTER BIRTH 329 



At birth the conducting fibres for the sensory areas of the 

 cerebral cortex are not all medullated. The tracts for the 

 visual centre develop their sheaths at the time of birth, while 

 those of the auditory area are developed after birth. The 

 fibres of association develop about three months after birth. 



As to the development of the senses the following facts 

 may be stated in regard to sight. During the fifth week, 

 fixation, associated movements of the eyes, closure of the 

 eyelids when the macula lutea is illuminated, and accommo- 

 dation take place, Only during the fifth month is there a 

 development of orientation of the visual field and closure of 

 the eyelids when the periphery of the retina is illuminated. 

 Until the fifth month the eccentric visual sensations are not 

 utilized. The child, therefore, appears as if it had an ex- 

 tremely limited visual field. An object upon which the gaze 

 is fastened is, during the fifth month, followed by the eyes, 

 but moving objects upon which the gaze is not fixed do not 

 call forth, during the first period, fixation of the eye. At 

 first the infant does not see the objects as solid objects and it 

 lacks all judgment of size and distance. (The child reaches, 

 e.g., for the moon.) It is also asserted that at birth the 

 .sense of color is absent, and that this begins to develop during 

 the sixteenth month and is completely developed in the fifth 

 or sixth year. The color sensations are first developed at 

 the centre and later on at the periphery of the retina. 



The other senses function immediately after birth, but it 

 is said that the sense of hearing is then incompletely devel- 

 oped ; this corresponds to the imperfect development of the 

 tracts of the auditory centre in the new-born. 



The change from infancy to childhood is marked by the 

 first dentition. The first teeth, the so-called milk-teeth, 

 develop in the following order: 



Between seventh and eighth month the lower central in- 

 cisors. 



Between eighth and tenth month the four upper incisors. 



Between twelfth and fourteenth month the four small 

 inner molars and the two lower outer incisors. 



