86 



THE EAR. 



which when traced back is found to turn downwards in the posterior wall of the 

 tympanum, sometimes opening at the base of the cranium by a small aperture just 

 in front of the stylo-mastoid foramen, and connected at one or two places with the 

 descending part of the aqueduct of Fallopius. A small bony spiculum often connects 

 the end of the pyramid with the upper part of the promontory (see fig. 93 B, 23). 



Anterior boundary. Canal for the tensor tympani. The anterior 

 extremity of the tympanum is narrowed by the gradual descent of the roof, 

 and is continued into the inner orifice of the Eustachian tube (fig. 85). 

 Above the commencement of this is the small (2 mm. diameter) canal which 

 lodges the tensor tympani muscle. This canal, which is lined by a fibrous 

 membrane, is about half an inch (12 mm.) long, and it opens immediately in front 



external 



pterygoid 



plate 



internal 



pterygoid 



plate 



Eustachian 

 cartilage 



Eustachian 

 cartilage 



carotid 

 foramen 



jugular 

 foramen 



posterior 

 condylar 

 foramen 



palate bone 



f"- nasal fossa? 



scaphoid 

 fossce 



foramen 

 ovale 



foramen 

 lacerum 



occipital 

 condyle 



foramen 

 magnum 



Fig. 94. BASE OF THE SKULL, SHOWING THE POSITION OP THE EUSTACHIAN CARTILAGE. 

 (Modified from Testut.) (E. A. S.) 



of the fenestra ovalis, surrounded by the expanded and everted end of the 

 cochleariform process, which separates it from the Enstachian canal. The bony 

 septum between the two canals is often incomplete, so that in the macerated bone 

 they may appear as a single large canal partly subdivided by a thin osseous 

 partition. 



In the recent state the fibrous sheath of the tendon is expanded over the end of the canal, 

 so as to impart to it a conical shape (see fig-. 101 . tf). 



The Eustachian tube (sdlpinx) (fig. 85, 4) is a canal about 86 mm. 

 (1'5 inches) long and from 2 mm. to 4 or 5 mm. diameter, bounded partly by bone, 

 partly by cartilage and fibrous membrane, which leads from the cavity of the 

 tympanum to the upper part < of the pharynx. From the opening into the 

 tympanum (ostium tympanicum) it is directed forwards and inwards, at an angle of 

 about 45 degrees with the sagittal plane, with an inclination of about 30 degrees 

 downwards from the horizontal ; the downward inclination is slightly greater in the 



