212 



Anatomy of Skeleton 



from its bony opening. This portion of the tube is fastened by firm connective tissue 

 to the bone in the neighbourhood. 



The minute foramen that transmits the tympanic branch of the glosso-pharyngeal 

 nerve is on the bony crest between the arterial and venous foramina : the ninth nerve, 

 with the tenth and eleventh, lies in the front part of the jugular fossa, therefore just 

 behind the artery immediately below the base of the skull. The little petrous ganglion 

 of the ninth nerve lies in contact with the petrous on a small triangular surface of bone 

 by the antero-internal angle of the jugular fossa (Fig. 173). 



The petrous and tympanic plate meet along the outer margin of the jugular fossa, 

 and thus a crest is formed here by their growth in apposition : this makes the vaginal 

 process, so that the base of the styloid process and its vaginal sheath may be said to 

 project immediately external to the jugular fossa or foramen. 



A small foramen for the posterior auricular branch of the vagus is near this outer 



FIG. 174. To show how the ductus endolymphaticus comes to lie 

 on the inner side of the petrous, i is a section like Fig. 172, 

 and in it the early brain N. is seen with its membranes M. 

 Outside this the otocyst has come to rest on the roof of the 

 tubo-tympanic recess of the pharynx, having descended from 

 the dorso-lateral surface where the notch is seen ; its track 

 is marked by a, the part which disappears, and d.e., which is 

 the drawn-out stalk of the otocyst and becomes the ductus. 

 In 2 it is seen that the growth of the upper and back part of 

 the otocyst to form semicircular canals has, with the accom- 

 panying growth of the capsule (petrous), left d.e. on the inner 

 side, while the growth of N. has pushed M. against and above 

 the capsule so that the ductus now lies between bone and 

 membrane. Sq., plane of squama. 



part : in the foetus the nerve is not enclosed in the bone, but is taken in later as the 

 tympanic plate and petrous increase in size. 



The remainder of the lower surface extends backwards and outwards behind the 

 jugular fossa to form the basis of the mastoid region. The thick inner margin of this 

 region is also slightly overlapped by the occipital, so that the articular area immediately 

 behind the fossa looks downwards as well as inwards : this is the part that in the first 

 half of life is separated from the outer end of the jugular process of the occipital by a 

 thin plate of cartilage. 



The lower aspect of the mastoid portion shows a deep groove for Digastric just 

 internal to the mastoid process, and part of a groove for the occipital artery internal to 

 this along the sutural margin. The artery runs to the base of the skull along the deep 

 surface of the posterior belly of the muscle, so its groove begins just internal to that for 

 the Digastric. At the front end of the digastric groove is the stylo-mastoid foramen. 



