SUBCRANIAL AND FACIAL STRUCTURES. 7.VJ 



■of the pterygo-palatine wall of tlie nose and mouth, and the 

 superior maxillary bone. The nasal pits or primary nasal depres- 

 sions, which extend themselves afterwards into the nasal fossae, and 

 remain permanently open to the exterior, are formed by a depres- 

 sion of the surface of the epiblast in the anterior prolongation of 

 the head as constituted by the ends of the trabeculas, and the harder 

 structures of the septum and walls of the nasal cavities, as well as the 

 turbinated structures on which the olfactory nerves are distributed, are 

 all derived from the anterior parts of these trabecule ; — a mesial union 

 giving rise to the nasal septum, while lateral parts circling round the 

 nasal pits, form the alar enclosures of these depressions (see figs. 537 

 and 538). 



The second preoral subcranial plates have received the name pterygo- 

 palatine from the nature of their deeper connection with the bar in 

 which the pterygoid and palate-bones are afterwards formed. These 

 enclose the posterior nasal apertures, and advancing from the two sides, 

 at last meet each other in the palate, and in front meet the pre-maxil- 

 lary process to complete the palate and upper jaw. 



But these are only the deeper parts of the structure out of which the 

 upper jaw is formed, there being on the surface of the head, and behind 

 the depression of the eye on each side, a bulging process known as the 

 superior maxillary process, in which the upper jaw and malar bone 

 are formed, and which has externally the appearance of bending round the 

 angle of the mouth in continuity with the mandibular or inferior maxillary. 



The formation of the superficial parts of the face, as seen from 

 before, may be described as follows, viz. : — In the middle, there 

 descends in what now forms the region of the forehead, a mesial 

 portion, the fronto-nasal plate, which forms the integument of the 

 nose, as far as the inside of the nostrils, and the columella of the nose, 

 together Avith the mesial part or lunula of the upper lip, that is, all 

 the part lying inside the depression of the nostrils. On the outside of 

 these depressions, a short lappet surrounds the orifice of the nostrils, 

 as wings forming the external nasal processes. It is towards the 

 outside of these last plates that the ocular depression is situated, that 

 depression being thus interposed between the lateral or external nasal 

 plate,- and the maxillary plate, and forming the fissure which has been 

 tailed the ocular fissure, but which afterwards becomes more fetrictly 

 the lachrymal in its anterior part (see figs. 536 and 539). 



The great buccal aperture now passes across the face, having the 

 middle and external nasal with the superior maxillary plates above 

 and in front, and the inferior maxillary plates below and behind. It 

 will be remembered, however, that the cavity of the mouth is thrown 

 forward by the outward development of the subcranial plates, which 

 deepen more and more the buccal cavity as they grow outwards from 

 the primitive cranium. 



The POSTOKAL pairs of pharyngeal visceral plates are four in number ; 

 the first being that already mentioned as following immediately behind 

 the mouth, and forming the mandibular or inferior maxillary. 



At an early period of foetal life in the human foetus, and in that of 

 all vertebrates, plates of this description are found, but in the lower 

 vertebrates a greater number exists than in the higher. The number of 

 four pairs belongs to man in common with all the non-branchiate 

 vertebrates. Behind each of these plates there are formed from within, 



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