_Mandibular clefts_ are due to the non-union of the separately developed lateral segments of the mandibular process—a deformity which must result from an earlier interference with the normal conditions of development than any of the others. Prof. Wölfler has pointed out that at the period when the branchial arches are being formed, the aortic bulb lies between their ventral extremities, reaching up even as high as the mandibular processes. If from any cause the retrogression of the heart and aortic bulb into the thorax is interfered with, then non-union of the visceral arches may result, and even a cleft mandible may thus be caused. This ingenious theory is stated only to apply to the more severe cases.[62] _Cleft palate_ is due to non-union of the palatal outgrowths of the maxillary processes. When the cleft extends beyond the anterior palatine canal it may pass along any of the intermaxillary sutures, but usually between the endo- and meso-gnathion on one or both sides. Inasmuch as the palate closes normally from before backwards, it is most common to find the deficiencies at the posterior rather than at the anterior end. CHAPTER IV. THE ANATOMY AND PHYSIOLOGY OF HARELIP AND CLEFT PALATE. _Harelip—Effect of labial muscles on deformity—Structure of os incisivum and labial segments._ _Cleft palate—Arrangement and action of muscles—Shape of bony segments—Associated irregularity in shape of skull—Physiological effects in nutrition, articulation, &c._ The short description of these congenital conditions given in Chapter I must be now supplemented by a little more exact account from an anatomico-physiological point of view. With regard to harelip, if unilateral, but little remains to be said, except to emphasize the fact that the deformity is not altogether due to loss of substance, but to a considerable extent to the unbalanced action of muscles, the equilibrium of which has been disturbed by the fissure. Thus the orbicularis oris, which should have a sphincter-like action, has its continuity interrupted, so that when contraction occurs, the effect will be to widen and evert the edges of the cleft. The muscles acting upon the corners of the mouth, moreover, will tend to exaggerate the deformity, and thus all such actions as laughing and crying will have a similar result. The margins of the cleft are rounded, and the red mucous border of the lip passes up for a variable distance on either side, but does not extend to the apex except in very slight fissures. The upper part of the cleft in the more serious forms has its margin formed of skin, a fact which must not be overlooked in planning an operation for its cure, and which will be again alluded to in the next chapter.