622 



EMBRYOLOGY. 



cartilaginous septum of the nose in the third month. The two 

 lamellffi afterwards fuse, the cartilage between them disappeariag. 



II. Bones of the Visceral Skeleton. 



The remaining bones of the head, which have not been mentioned 

 hitherto, belong to the visceral skeleton, some of them being 

 primordial, others covering bones. 



The hyoid bone and the auditory ossicles (perhaps also the thyroid 

 cartilage) are primordial parts; they are characterised by very 

 diminutive size and occupy a very subordinate position in comparison 

 with the enormously developed covering bones. The hyoides begins 

 toward the end of embryonic life to ossify at several points. The 

 auditory cartilages acquire from the periosteum as early as the fourth 

 month a bony investment, within which here and there remnants of 

 cartilage persist even in the adult. According to recent researches 

 the malleus is a compound skeletal piece. The long process is de- 

 veloped as a covering bone on that part of Meckel's cartilage which 

 penetrates between petrosal and annulus tympanicus. While the 

 cartilage undergoes degeneration, the covering bone fuses with the 

 larger, primordial part of the malleus. It probably corresponds 

 with the OS angulare of lower Vertebrates. 



The covering bones of the visceral skeleton, the maxillare superius, 

 palatinum, pterygoideum, zygomaticum, and maxillare inferius, are 

 developed in the vicinity of the mouth-opening in the connective 

 tissue of the superior and inferior maxillary processes. 



The maxillaria superiores are a complex of two pairs of bones, 

 which indeed remain separate in most Vertebrates. One pair is 

 developed on the two superior maxillary processes laterad of the 

 cartilaginous nasal capsule. The other pair appears in the eighth or 

 ninth week, according to Th. Kolliker's detailed investigations, 

 upon the part of the frontal process that lies between the nasal 

 orifices. It corresponds to an actual paired intermaxillary (pre- 

 inaxillare), and subsequently encloses the fundaments of the four 

 incisors. 



The two intermaxillaries in Man early fuse with the fundaments 

 of the two superior maxillaries, the two membranoiis superior 

 maxillary processes having previously united with the inner nasal 

 processes. The boundary between maxillary and intermaxillary is 

 indicated on the crania of young persons by a suture-Kke place 

 (sutura incisiva), running transversely outward from the foramen 

 incisivum, which is occasionally retained even in the adult. 



