While these modifications are proceeding in the immediate coverings of the brain, additional visceral elements are being added on the under aspect of the base. These are derived from the visceral arches that constitute the floor of the primitive pharynx : it is evident (Fig. 211) that the outer and upper parts of the mesoderm of these arches must come into relation with the ventro-lateral part of the covering of the brain, 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 mandibular (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. 2X1, 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. When 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 possible 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 cartilaginous base, making it continuous with the prechordal portion of this base. The capsule 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.
We find, then, that the skull can be said to be made up of bones which may be divided into classes according to their source of origin. Thus we have :-
(a) Bones of the Cranial Capsule.
(b) Special Sense Capsules.-The ethmoid and petrous in this group can perhaps be added as a subdivision to (a), but the vomer, nasal, and lachrymal may be placed in this class as formed in membrane on the capsule, and the premaxilla may perhaps be put provisionally with them.
(c) Visceral Skeleton.-This includes (1) membrane bones formed in the arches, mandible, maxilla, malar, palate, internal pterygoid plate ; (2) bones formed in cartilaginous bars of the arches-styloid process, ossicles of ear.
(d) The Tympanic Plate.-The value of this is uncertain, and, although it is often referred to as homologous with certain ossifications found in lower types of jaw, it would be safer from our present point of view to place it simply in a class by itself without further remark on its morphology.
The bones of the cranial capsule can be further subdivided into cartilage and membrane bones : the membrane bones are the frontal, parietal, squamous temporal, and interparietal part of occipital, with a large part of the great wing of sphenoid. The chondro cranium underlies that part of the brain that expands least, and the membrane bones cover the portion that grows most rapidly.