6. That the os incisivum consists merely of the two united endognathia, and normally carries only the two central incisors. Any additional tooth is not the normal lateral incisor, but an accessory one, probably due to the reappearance of an old suppressed member.
7. That any precanine existing on the outer side of the cleft is the normal lateral incisor, springing from the mesognathion; but that the latter portion of bone is rarely seen as a separate entity in human skulls possessing such deformity, from early obliteration of the suture between it and the maxilla, and that it is often badly developed and the lateral incisor stunted or undeveloped from defective vascular supply.
Having entered thus fully upon the question as to the situation of the cleft in alveolar harelip, we must now turn to the consideration of the morphological position of the oblique facial clefts, and their relations to the above.
It has been already pointed out that they commence at about the same spot in the lip margin as ordinary harelip, and thence run upwards and outwards clear of the ala nasi towards the centre of lower eyelid, following along the line of the naso-orbital fissure. Such a condition, coupled with the developmental facts already mentioned, viz. the existence of four segments in the intermaxilla and their relations to the internal and external nasal processes, suggests the following conclusions:
1. That the situation of the cleft in the lip margin is in all probability between the internal nasal and maxillary processes. The truth of this proposition depends on whether or not the external nasal process has any share in the formation of the lip. His, in his diagrams ([Fig. 30]), seems to indicate that it does not, whilst Biondi[61] claims that it does. The fact that these facial clefts commence at about the same spot in the lip margin as do the clefts in harelip seems distinctly to point to the conclusion that the superficial portion of the external nasal process is limited to the development of the ala nasi, and of the soft parts immediately around it. And this opinion goes far to explain the dimpling of the skin around and the consequent definition of the ala nasi.
2. That the situation of the cleft in the alveolus is between the meso- and exo-gnathion, so that the first tooth on the outer side, if developed, will be the canine.
3. That the upper extremity of the cleft should typically be located at either the inner canthus or about the middle of the lower eyelid; but this is not always the case. The association with coloboma iridis is readily explained by an imperfect closure of the choroidal cleft which normally occurs at the lower and inner segment of the globe.
Median harelip in its two forms is readily explained. The true median cleft is due to the non-union of the two globular processes developed from the median portion of the fronto-nasal outgrowth. If prolonged between the bones, it occupies the position of the median inter-intermaxillary suture, and so passes between the two endognathia.