We have seen (p. 193) that the bone consists of four distinct morphological elements, squamous, petrous, styloid process, and tympanic plate, and decidedly the best way to become acquainted with the plan of build of this complicated bone is to obtain first of all an idea of the reason why these constituents of the bone come into relation with each other, and what position in regard to one another they must bear.
Fig. 172 is intended to illustrate these points schematically. The first drawing is a scheme of a transverse section through the head of an embryo, showing on the left side how the otocyst, from which the inner ear is formed, lies on the roof of the recess which runs out from the early pharynx and from which the Eustachian tube and tympanum are formed : the cartilages of the first and second visceral arches lie under the floor of the recess. This tubo-tympanic recess comes into contact with the surface at one point, x ; behind and in front of this point it lies some distance from the surface. The first arch cartilage, or Meckel's cartilage, comes up above the level of the pharynx in the interval in front of, and that of the second arch behind, the point of surface contact. The otocyst is surrounded by a mass of condensed mesenchyme : this chondrifies later and ossifies, making the petrous. The squama is a separate development in the tissues superficial to the rest : the right side of the first figure indicates the planes of the different structures, and it can be seen that when the upper end of a bar comes above the pharyngeal level, it must lie between the petrous and squamous, covered on the surface by the latter. No. 2 represents a view from the right side, in which the first and second cartilaginous bars are seen passing up in front of and behind the surface point x. The tubo-tympanic recess is directed upwards, outwards and backwards and slightly twisted, so that the petrous rudiment lies above, behind and internal to it : thus the upper end of the second bar is between the petrous and the recess.
The third figure represents these relations, with the recess drawn out and showing a dilated tympanic end with a narrower tubal part in front. The petrous projects back beyond the end of the recess. A wing is thrown out from the petrous over the tympanum : this is called the legmen tympani and is shown in No. 4. But the squama is in relation with the upper part of the outer surface of these structures, as shown in No. 5. A tympanic ring, incomplete above, and developed in superficial tissues, is also shown surrounding the surface area of the tympanum. Thus part of the tegmen and the petro-mastoid are exposed below the squama, and the ring is partly in contact with them. The second bar joins the petro-mastoid at the back of the tympanum, and can now be termed the styloid. The first bar has its upper end under cover of the tegmen, and it disappears below this : the upper end forms the malleus. So it is not seen in No. 5, but its former position is indicated. In No. 6 growth has led to the squama covering more of the tegmen, and the tympanic ring is beginning to broaden into a plate, so that the margin of the tegmen lies below and in front of it, between it and the squama, while it partly covers the upper part of the styloid below and behind. The lower jaw articulates with the part of squama that is covering the tegmen.
No. 7 shows the further progress to the adult state. A margin of tegmen is still seen between the articular cavity of the squama and the tympanic plate ; the plate has much increased in size, making the bony meatus and piling itself up against the styloid. Where the front border of the plate opposes the squama the " Glaserian fissure " is formed, so that the edge of the tegmen can be described as visible in the inner or lower part of the Glaserian fissure. The mode of formation of the mastoid process is also indicated, the back part of the squama being drawn down over the petro-mastoid : thus a " petro-squamous suture " is visible on the surface of the process.