It is difficult to determine just how far the sense organs have arrived in their development at the time when the infant begins its independent existence. That touch and those senses related to bodily discomfort, of which pain is most important, are operative from the first is shown by the occurrence of the reflexes described above, which are brought into action by those particular senses. There is good reason to believe that the sense of hunger comes into play within two or three days at the latest. The sense of thirst does not have much chance to reveal itself early in life, for with a diet exclusively liquid, and feedings separated by only a few hours, there is no reason why the child should become thirsty. As the feedings become less frequent, and particularly as solid food begins to be added, there is real danger that the child may be insufficiently supplied with water. Muscle sense and the equilibrium sense, if present at all, must be in a very imperfect condition at first. They seem to differ from the senses described thus far in that they depend on practice for their development. At any rate the bodily movements are largely aimless in the beginning, and it will be observed that the baby has the appearance of experimenting with its extremities, placing them repeatedly in particular positions and seeming to gain precision thereby. The eye movements, and especially those by which both eyes are focused on a single object, depend for their accuracy on the working of muscle sense. In the estimation of the parents a distinct mark of progress is registered the first time the baby follows a movement with its eyes. As soon as it does this accurately, and also brings both eyes to bear on any object, its muscle sense is known to be in efficient operation, so far, at least, as the eye muscles are concerned. Equilibrium sense first shows itself when real balancing motions of the body are made. The senses of taste and smell may be operative to some degree in very young children, but it is doubtful whether they have either the breadth or the acuteness that will be shown later in life. Recognition of disagreeable tastes or smells seems to appear earlier than appreciation of agreeable. This is in line with the general fact that the self-protective reactions are developed very early.