666 ANIMAL PHYSIOLOGY. 
to the head. Is this an example of correlated growth, to be 
explained by the predominance of the cerebrum ? 
Nervous System.—The nervous system, like all the others, is. 
highly sensitive; it reacts powerfully to moderate stimuli; its 
equilibrium i is more readily disturbed than that of any other. 
and, since to it belongs the work of guiding the metabolic pro- 
cesses of the various tissues, this peculiarity explains the readi- 
ness with which the health of the infant can be deranged or 
restored. Hence it follows that a prognosis in the case of in- 
fants must be unusually guarded. 
As has been already indicated, the cortical cells of the cere- 
brum, and other parts of the brain, are but indifferently devel- 
oped at birth; so that stimulation of the cerebral surface in 
young animals (though there is great difference in this respect) 
must not be expected to give precisely the same results as in 
adults. 
From the share that we now know the cortex of the cere- 
brum to take in the elaboration of probably all sensory im- 
pulses, it follows that in the infant all of the senses must be to 
a certain extent imperfect, even assuming that the peripheral 
mechanisms are as perfect functionally as in the adult, which 
is not likely. 
In some respects, however, the eye of the infant is more 
perfect. Its power of accommodation for near objects is won- 
derful, while at a very early age the pupil acts perfectly, and 
binocular vision is established. 
Touch is fairly developed, and probably also taste and 
smell; though as to the last two there is more doubt. On the. 
other hand, hearing in the infant.is very imperfect; power to 
discriminate between the pitch and quality of sounds is rudi- 
mentary; while appreciation of direction, which is largely the 
result of experience, is necessarily of the crudest. 
It is doubtful if the middle ear is properly pervious to air, 
on which its functioning depends greatly for some time after 
birth. But certainly, as regards the processes of the peripheral 
mechanisms of the senses, the child that has passed the years 
of infancy knows a perfection, to which he becomes more and 
more a stranger as years pass by. Later he will, in consequence 
of accumulating experience, make more out of his sensory 
data; his cerebral cortex will be more developed, both struct- 
urally and functionally. 
Maturity (Puberty).—Though most of the organs of the body 
continue to improve, and certainly the organism, as a whole, 
