Skull and Hyoid 209 



TEMPORAL. 



A bone consisting of a tabular or squamous part visible on the side of the skull, 

 but with its more important petrous part, carrying the organ of hearing, embedded in 

 the base of the skull. 



We have seen (p. 193) that the bone consists of four distinct morphological ele- 

 ments, 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 con- 

 tact. 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 



F.A. 14 



