On account of the early age of the infant, and for the reasons which have been given, the first dentition is more liable than the second to be ac companied by serious disturbances ; but even in cutting the second crop of teeth, digestive troubles are likely to occur, as will be afterwards de scribed.
The first dentition begins under normal conditions in the middle of the first year, and ends toward the beginning of the third. The eruption of, the milk teeth may, however, be anticipated or delayed through individual peculiarity, or some abnormal constitutional state. Thus, cases occasionally occur in which the child is found to have a tooth when he is born. Such teeth are usually sharp and hook-shaped, and are often loose, consisting merely of the crown of the tooth embedded in a fold of the gum. Henbch has described another variety of congenital tooth, which is firmly fixed in the socket. The tooth is destitute of enamel, and looks yellow, with a rough surface. Henoch attributes the eruption to a periostitis of the al veolar border, which pushes the rudimentary tooth outwards by swelling and exudation within the socket.
It is not uncommon for teeth to begin to be cut at the third or fourth month ; but in such cases the eruption of one or two teeth is usually fol lowed by a pause, and the continuance of the process is deferred until the usual age. In certain states of the constitution, dentition is early. Thus, children with tubercular tendencies, or who suffer from a syphilitic ca chexia, cut their teeth early, as a rule. In rickets, on the contrary, denti tion is always late, and in exceptional cases no tooth may appear until the end of the second or beginning of the third year. Ordinary malnutrition, when the child has not become rickety, does not interfere with the evolu tion of the milk teeth. In chronic diarrhoea, when the child is very weakly, and much wasted by constant purging, I have often noticed with surprise that the natural evolution of the teeth has been in no way retarded by the distressing complaint.
In an ordinary case the milk teeth appear in the following order : Lower central incisors, upper central incisors, upper lateral incisors, lower lateral incisors, first molars, canines, back molars. Of these the first should appear between the seventh and ninth month. At twelve months old the infant should have cut eight teeth, and the four first molars should be in process of evolution. He should cut his eye-teeth (canines) between the sixteenth and twentieth month ; and the whole number of the first -crop (twenty) should have pierced the gum soon after the end of the second year. The teeth are usually cut in pairs ; and after the completion of
each group there is usually a pause before the evolution of the next group begins.
The order given above, although that which most commonly obtains, is yet often departed from in children whose health is perfectly good. Many babies cut their teeth "cross," as it is called. The lateral incisors sometimes appear before the central front teeth ; the first molars may precede the lateral incisors ; the last molars may precede the canines ; and in a few instances I have seen a canine tooth cut before any of the first mo lars have appeared, but this last exception is a very rare one. Sometimes in rickety children, when dentition is greatly retarded, the first tooth to appear is one of the first molars. Thus, a rickety little boy under my care cut his first tooth—one of the first molars—at the age of two years. An other cut his earliest tooth—also a first molar—at fifteen months.
Although the full number of the milk teeth when dentition is completed is twenty, this number is not always reached. It may happen that certain teeth never appear at all. Thus, a little girl under my care, aged two years and nine months, was seen to have all the milk teeth except the Vivo upper lateral incisors. On the left side there was a narrow space re maining between the left middle incisor and the canine ; but in this space the gum was sharp, and there was no sign of a tooth. On the right side, the right central incisor and the adjoining canine were in contact. In the same way I have known the whole four canines to be absent. In some cases the peculiarity is a hereditary one. In a case which came under my notice the left lower lateral incisor was wanting in a little girl of two years old. The same incompleteness of the milk teeth had occurred in the mother. This lady had three other children—all boys—whose early den tition had presented no deviation from the normal type. It is certainly curious that the irregularity which had occurred in the mother should have been reproduced in the only one of her children whose sex was the same as her own. It is important to be aware that incompleteness of the first crop of teeth does not necessarily imply that a similar irregularity will be met within the second. Mr. Tomes, in his work on dental surgery, refers to the case of a little girl who cut none of her milk teeth, but in whom the permanent set appeared as usual. Sometimes, instead of too few, too many milk teeth are developed. A little girl between two and three years old lately came under my notice who had five perfect incisors in the lower jaw.