2IO 



Anatomy of Skeleton 



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. 



As soon as the student comprehends the way in which the complete bone is put 

 together, he can proceed to recognise the various parts and their necessary relations 

 in the adult bone. Look at the bone from below ; the Glaserian fissure (Fig. 173) is 

 easily recognised at the bottom of the hollow where the jaw articulates. Behind it 

 is the tympanic plate forming the bony meatus ; in front of it is the articular surface 

 of the squama, while the edge of the tegmen appears in the inner part of the fissure. 

 The styloid process has the inner and back part of the plate heaped up against its outer 

 and front aspect, constituting the vaginal process. On the inner side of the styloid and 

 tympanic plate the mass of the petrous is directed forwards and inwards. It is clear, 

 from study of Fig. 172, that the Eustachian tube must emerge from the bone between 



FIG. 173. Lower aspect of temporal bone. The arrow passes in at the Eustachian opening 

 and out at the external meatus ; such a passage would only be possible when the tympanic 

 membrane is absent. M., mastoid process. 



petrous and tegmen, separated from the squama by the tegmen, and above and internal 

 to the inner part of the tympanic plate ; examine the bone (Fig. 173), and the opening 

 for the tube can be found in this position, in the angle, roughly speaking, between the 

 petrous and squama, but really separated from the latter by the thin lamina of the 

 tegmen. There is really a double opening here, both leading to the bony tympanum and 

 separated by a very thin layer of bone, the upper canal for Tensor tympani and the lower 

 for the Eustachian tube. Now follow the margin of the squama where it is applied to 

 the superficial aspect of the petrous bone, comparing the complete structure with the 

 schemes in Fig. 172. Starting above the external meatus, the margin can be traced 

 down in the Glaserian fissure, being here a squamo-tympanic suture, but in the inner 

 and lower part of the fissure it becomes petro-squamous. At the inner end of this 

 part it turns up on the upper surface, still separating the squama from the tegmen 

 (Fig. 162), and passes back along here as the (upper) petro-squamous suture to the 

 petro-mastoid region : it is generally obliterated wholly or in part in this region in the 



