The os incisivum has usually a larger superficial area than this “philtrum” of the upper lip, and hence protrudes beyond it in all directions. It forms a projecting tubercle, covered by smooth mucous membrane on its under side, with the central portion of the upper lip attached anteriorly. In a young child it consists of two little portions of bone, imperfectly united together, which in the fœtus are represented by two cartilaginous nodules, mobile on each other, and within each a separate ossific centre; in other words, it is formed by the two endognathia. Inside are found the rudiments of a variable number of teeth; ordinarily in a child’s os incisivum, operated on at the usual age (viz. one to three months), one finds on laying it open the rudiments of four teeth, the temporary and permanent central incisors, arranged in pairs, one above the other. Occasionally, as has been already mentioned (p. 54), one finds evidence of the development of another incisor on one or both sides of the projecting tubercle, and directed towards the cleft; but such is usually imperfectly developed and stunted. In fact, amongst all the ossa incisiva removed by Sir William Fergusson and now preserved in King’s College Museum, but few show any traces of the additional incisor, whilst the common arrangement is to find only the two central teeth (Fig. 41). In no case is there any evidence of the existence of more than two bony segments. The anterior wall of the bone is always badly developed, and most commonly when displaced the growth of the whole projection is somewhat impeded, so that it is smaller than in the normal condition. Its position may vary, being occasionally but little displaced anteriorly, though in consequence of its slight basis of support, viz. the antero-inferior extremity of the vomer, it is generally mobile; bands of muco-fibrous tissue are occasionally seen passing from it to the maxilla under such circumstances. Every variety of anterior displacement is met with, until the severest forms alluded to above are reached. If operative interference be delayed until late in life, the vomer becomes dense and hypertrophied, and the junction with the os incisivum much firmer, increasing the subsequent difficulties and dangers of treatment. More exact details as to the dentition in cases of alveolar harelip have been already given and discussed in a former chapter (p. 51). It is interesting to note here, however, that the temporary incisors, both in the intermaxilla and lower jaw, have a tendency to appear earlier than usual; I have many times seen incisors in such cases erupted at birth. * * * * *