DEVELOPMENT OF THE OSSEOUS SYSTEM. 



879 



goid process, while the non-cartilaginous and previously formed inner lamina arises from the 

 superior maxillary process of the first branchial arch. During the first half of foetal life these 

 centres unite as far as the great wings ; the dorsum sellse and the clinoid process, as far as the 

 synchondrosis spheno-occipitalis, are still cartilaginous, but they ossify at the 13th year. 



HI. The pre-sphenoid at the 8th month has two centres in the small wings and two in the body. 

 At the 6th month they unite, but cartilage is still found within them even at the 13th year. 



IV. The ethmoid has a centre in the labyrinth at the 5th month, then in the 1st year a centre 

 in the central lamina. They unite about the 5th or 6th year. 



V. Amongst the membranous bones are the inner lamina of the pterygoid process (one 

 centre), the upper half of the tabular plate of the occipital (two points), the parietal bone (one 

 centre in the parietal eminence), the frontal bone (one double centre in the frontal eminence), 

 three small centres in the nasal spine, spina trochlearis and zygomatic process, nasal (one 

 centre), the edges of the parietal bones (one centre), the tympanic ring (one centre), the 

 lachrymal, vomer, and intermaxillary bone. 



The facial bones are intimately related to the transformations of the branchial arches and 

 branchial clefts (fig. 671). The median end of the first branchial arch projects inwards from 

 each side towards the large oral aperture. It has two pro- 

 cesses, the superior maxillary process which grows more 

 laterally towards the side of the mouth, and the inferior 

 maxillary process, which surrounds the lower margin of 

 the mouth (fig. 665, IX). From above downwards there 

 grows as an elongation of the basis cranii the frontal pro- 

 cess (s), a broad process with a point (y) at its lower and 

 outer angle, the inner nasal process. The frontal and the 

 superior maxillary processes (?) unite with each other in 

 such a way that the former projects between the two latter. 

 At the same time there is anchylosed with the superior 

 maxillary process the small external nasal process (n), a 

 prolongation of the lateral part of the skull, and lying 

 above the superior maxillary process. Between the latter 

 and the outer nasal process is a slit leading to the eye (a). 

 Thus the mouth is cut off from the nasal apertures which 

 lie above it.' But the separation is continued also within 

 the mouth ; the superior maxillary process produces the 

 upper jaw, the nasal process, and the intermaxillary pro- 

 cess (Goethe) the latter is present in man, but is united H( ; ad of embryo rabbit of 10 days 

 to the upper jaw. The intermaxillary bone, which in 

 many animals remains as a separate bone (os incisivum), 

 carries the incisor teeth. At the 9th week the hard palate 

 is closed, and on it rests the septum of the nose, descend- 

 ing vertically from the frontal process. The lower jaw is 

 formed from the inferior maxillary process. At the circum- 

 ference of the oral aperture the lips and the alveolar walls 

 are formed. The tongue is formed behind the point of the 

 union of the second and third branchial arches (His); while, 

 according to Born, it is formed by an intermediate part between the inferior maxillary processes. 



These transformations may be interrupted. If the frontal process remains separate from the 



Fig. 671. 



( x 12). a, eye ; at, atrium or 

 primitive auricle of heart ; b, aortic 

 bulb ; K', K", K"', first (mandi- 

 bular), second (hyoid), third (1st 

 branchial) visceral arch ; m, mouth; 

 s, superior, and u, inferior max- 

 illary process ; s, mid-brain ; v, 

 part of head and fore-brain ; v, 

 ventricle of heart. 



W+e 



Fig. 672. Fig. 673. 



Fig. 672. Hare lip on the left side. Fig. 673. Inner view of the lower jaw of an embryo pig 

 3 inches long ( x 3^). rnk, Meckel's cartilage ; d, dentary bone ; cr, coronoid process ; ar, 

 articular process (condyle) ; ag, angular process ; ml, maileus ; mb, manubrium. 



superior maxillary processes, then the mouth is not separated from the nose. This separation 

 may occur only in the soft parts, constituting hare-lip (fig. 672) ; or it may involve the hard 



