The space between the segments of the lip is usually triangular in shape, and like an inverted V; it may or may not communicate with the nostril. In alveolar harelip the alveolus is cleft, as has been already described, along the endo-mesognathic suture; but the floor of the nose is not necessarily implicated. The line of fissure in many instances passes through the maxillary attachment of the depressor alæ nasi, and the absence of the controlling influence of this muscle is an important element in the production of the broad flattened condition of nostril such a common accompaniment of this deformity, thus affording an explanation of the nasal distortion in cases where the alveolus is intact. If that structure be also implicated, then the floor of the nose will be deficient to a greater or less extent, and the tendency of the nostril to fall away increased. On raising or making tense either segment of the cleft lip, the existence of strong reflections of the mucous membrane or frænula will become evident, in addition to the normal mesial frænum; these are sufficiently firm to limit the range but not to antagonise completely the action of the muscular contractions already alluded to. Moreover, unless freely divided by undercutting they will effectually prevent by their tension the parts being brought into a state of easy apposition, so necessary in order to gain primary union. In bilateral or double harelip the maxillary segments on either side correspond in every particular with the outer segment in a unilateral cleft; but the central portion which is continuous with the columna nasi deserves special notice. It is usually ovoid in shape and stunted, appearing as if shrunken upwards from the absence of lateral support; its breadth and length are nearly equal, and there is a small portion of the red labial margin at the lower part. It is attached on its deep aspect to the os incisivum by firm muco-fibrous frænula, and in aggravated cases it appears to project amalgamated with the columna from the tip of the nose, forming the proboscis-like appendage already illustrated (Fig. 8). [Illustration: FIG. 41.—Os incisivum, consisting of two lateral bony segments, each bearing an incisor. (_Fergusson._)]