206 THE ORGANS OF SPECIAL SENSE 



tympanic cavity. Its roof is formed by the tegmen tympani 

 (p. 200) and \\sfloor by the jugular fossa (p. 200). Anteriorly, 

 it is related to the middle ear. Posteriorly, a thin plate of 

 bone alone separates it from the transverse (lateral) sinus, as 

 the vessel descends in its groove on the mastoid portion of the 

 temporal bone (p. 114). In its medial wall the lateral semi- 

 circular canal is embedded, while the facial canal turns 

 downwards in the medial wall of the aditus. Laterally, the 

 antrum is related to the lateral surface of the skull immedi- 

 ately behind the upper part of the external acoustic meatus. 

 In the infant this wall is only about one-eighth of an inch 

 thick ; by the sixth year, it has increased to a quarter of an 

 inch, and, in adult life, it varies from a half to three-quarters 

 of an inch in thickness. 



Suppurative disease in the tympanic (mastoid) antrum is a 

 fertile source of intra-cranial abscess. When the infection 

 spreads in an upward direction, the temporal lobe of the brain 

 is involved, but it may spread backwards, causing thrombosis 

 of the transverse (lateral) sinus. This vessel receives many 

 tributaries from the cerebellum, and the infection may spread 

 along them, ultimately giving rise to a cerebellar abscess. 

 Spread in a medial direction involves the internal ear, and 

 chronic progressive inflammation of the labyrinth (p. 90) is 

 the result. 



The Internal Bar consists of a complicated, closed, mem- 

 branous tube, termed the membranous labyrinth, which is 

 situated in the osseous labyrinth, a large space in the interior 

 of the petrous temporal. The anterior part of the osseous 

 labyrinth is known as the bony cochlea, the middle part is 

 termed the vestibule, while the posterior part constitutes the 

 osseous semicircular canals. 



The anterior extremity of the membranous tube is spirally 

 coiled to form the cochlea, and the posterior end of the cochlea 

 opens into a small sac which is known as the saccule. The 

 posterior extremity of the tube is arranged to form three semi- 

 circular ducts, set, respectively, in vertical, frontal (coronal), 



