264 Anatomy of Skeleton 



which lies in close association with the roof of the pharynx. In this way the upper 

 ends of certain of the cartilaginous bars formed in these arches are taken up into the 

 skull, while other bones are developed in membrane in the arches and form the facial 

 portion of the skeleton ; these two sets of structures constitute together the visceral part 

 of the skull. It is only in the first three arches that a dorsal moiety of each bar is 

 formed, and in the case of the third bar this quickly disappears, so that only the upper 

 ends of the first two bars are finally represented in the skull ; these form (see Fig. 172) 

 the ossicles of the ear and the styloid process. 



The facial skeleton is developed secondarily from the mesenchyme of the mandi- 

 bular (first) arch. This arch gives a maxillary process forward on each side of the mouth 

 cavity, below the anterior part of the cranial capsule and the eye, and on the sides of 

 the nasal region : the arrangement is shown in Fig. 211, which also indicates the general 

 formation of the face by the arch and its maxillary process. The lower jaw ossifies 

 in the mesenchyme of the first arch itself, and in its maxillary process are formed the 

 maxilla, palate, internal pterygoid plate, and malar. 



The nasal capsule is the cartilaginous wall of the nasal fossae, surrounding them 

 above and at the sides and giving a cartilaginous partition between them. Its anterior 

 part is undoubtedly developed round the early olfactory pit, but there is some reason 

 to suppose that its hinder part is formed from the maxillary mesenchyme, a view which, 

 inter alia, renders intelligible the formation of the palate bone on the inner side of the 

 capsule while the maxilla is developed external to it. 



The early nasal pits are placed just above the fronto-nasal process, and these small 

 cavities gradually extend upwards and backwards, their cartilaginous capsules forming 

 round them as they extend, so that ultimately they come into their final positions, and 

 the capsules are flush with the presphenoid above. Then the spheno-ethmoidal plates 

 grow out along the orbitosphenoidal processes. The back of the cartilaginous septum, 

 however, is associated as an early condensation with the central part of the basal 

 cartilage, and the value of this portion may differ from that of the rest, \\~hen formed, 

 the capsule surrounds the cavities, and has projections and spaces within it which map 

 out the ethmoid, while the lower edges of its side walls turn in and fo m the basis of the 

 inferior turbinals. 



It would be better, perhaps, for us at present to look on the nasal capsule simply 

 as a sense-capsule developed round an organ of special sense, without regard to the pos- 

 sible values of its different parts, and to speak of the ethmoid, lachrymal, nasal, and 

 vomer as formed in connection with the capsule, the first preformed in cartilage and the 

 others ossified in membrane on the surface of the cartilage. Perhaps the premaxilla 

 if such a bone has a separate existence (see p. 235) in the human skull should be placed 

 in the same class, as it is developed in the fronto-nasal process made by the fusion of the 

 inner walls of the two olfactory pits.* 



By looking on the nasal capsule as a special sense-organ we put it for the moment 

 on the same level as the periotic capsule and can thus bring its upper surface, the part 

 that shows between the orbital plates of the frontal, into the conception of the cartila- 

 ginous base, making it continuous with the prechordal portion of this base. The cap- 

 sule of the eye does not become cartilaginous in the human skull, and is probably 

 represented by the sclerotic. 



* It has been suggested that the maxillary mesenchyme invades the frontal-nasal process from the sides 

 and provides the tissues of the region, with the exception of the portion of the process exposed behind as 

 the " incisive papilla." Although this view explains the nervous supply, etc., of the parts, its embryological 

 support is so doubtful that it would be wiser to look on the " premaxilla " as being possibly in line with some 

 other bones round the nasal capsule, without regard to its more detailed morphological value as indicated 

 in development. 



