Skull and Hyoid 211 



adult bone, but can be picked up again on the outer side of the mastoid process, running 

 downwards and forwards (Fig. 175) towards the apex, near which it turns up once 

 more and runs round the meatus, becoming s^uamo-tympanic. If the squama and 

 tympanic plate were removed, the tympanic cavity would be exposed, partly covered 

 by the Tegmen tympani, as can be understood from the schemes in Fig. 172. Thus the 

 cavity would have its inner wall made by the outer side of the petrous, its outer wall by 

 the tympanic ring and the membrane which this holds in position, its roof by the 

 tegmen, and the ossicles would be enclosed between these structures : we have already 

 seen that the upper end of Meckel's cartilage forms the malleus, the stapes is developed 

 from the upper end of the styloid bar, and the incus is made from a condensation 

 continuous with that of the malleus. The backward extension of the cavity that 

 forms the mastoid antrum goes back beyond the region of the ring and Membrana 

 tympani, so that its outer wall is formed by squama. 



The outer surface of the petrous portion, where it is not in relation with the tym- 

 panum and antrum, has the squama applied to it. The styloid process appears on 

 the surface between the tympanic plate and the petrous : the plate in its growth 

 spreads along the process and thus makes the " vaginal process," which therefore is 

 outside and in front of the styloid process. 



The student must become thoroughly familiar with these general relations of the 

 parts of the temporal bone to each other, working them out from different points of 

 view without troubling himself about the numerous foramina, etc., that he observes 

 on the bone, before proceeding to study the specimen in detail, for only by this means 

 will he be able to understand it and have the key to the many-sided intricacies of 

 its structure. 



The petrous part of the temporal is somewhat wedge-shaped, with its apex projecting 

 forwards and inwards and its base, cut obliquely, represented by the thick petro- 

 mastoid region of the bone. It has four sides : (a) outer or anterior, covered by squama 

 save where it forms the inner wall of tympanum ; (b) lower, seen on the basal surface of 

 the skull ; (c) upper ; and (d) inner or posterior. Dura mater covers (c) and (d), which 

 are respectively seen in the middle and posterior fossae of the cranial cavity. The apex 

 fits in between the great wing of the sphenoid and the region of the basi-sphenoid and 

 basi-occiput. The lower surface is really infero-internal ; compare the separate bone 

 with the complete skull, and it is seen that the inner part of the surface is overlapped 

 by the edge of the occipital, the remainder being visible from below. Thus we find an 

 articular surface internally, coated by a thin layer of cartilage in the recent state, for 

 the occipital, and behind this a jugular fossa, completed by the occipital, and converted 

 into the jugular foramen. In front of the fossa, on the exposed part of the surface, is 

 the rounded opening of the carotid canal that receives the internal carotid and the ramus 

 caroticus of the superior cervical ganglion. 



The artery is taken into the petrous bone secondarily ; it is at first below the 

 otic capsule, as shown in the first two drawings in Fig. 172, running forward over the 

 tubo-tympanic recess. Later, as the ossification spreads, it encloses the artery, but the 

 vessel retains its relation to the tubo-tympanic region (see later, p. 226). 



The jugular fossa is frequently very deep, lodging the " bulb " of the vessel : the 

 plate of bone that makes its floor is concerned, with the tympanic plate, in forming 

 the floor of the tympanic cavity. 



In front of the carotid opening and rather internal to it the surface is related to the 

 lateral recess (fossa of Rosehmuller) of the pharynx, and some fibres of Levator palati 

 arise here : immediately outside this is the outer surface of the bone, and here the 

 cartilaginous part of the Eustachian tube is running downwards, forwards and inwards 



142 



