MEMBRANOUS LABYRINTH. 469 



In the tipper Fossa are two foramina ; the anterior, which 

 is the largest, serves to transmit the Portio dura, a part of the 

 seventh pair, to be hereafter described, which passes through 

 the petrous bone to the face. The posterior foramen forms a 

 pit with a cribriform bottom, which admits nervous fibrillae to 

 the vestibule. 



The anterior part of the Inferior Fossa is also cribriform ; its 

 perforations lead to the cochlea ; one of them, which passes 

 through the modiolus to the infundibulum, is larger than the 

 rest. The posterior part of this fossa is occupied by foramina, 

 which pass to the vestibule and semicircular canals ; but they 

 are not so numerous as those which lead to the cochlea. 



' Contents of the Labyrinth. 



This interesting cavity is lined throughout by a delicate 

 membrane. It contains sacs and tubes, and a plexus of deli- 

 cate nerves, which constitute a soft labyrinth within that which 

 is composed of bone. 



In the cavity of the Vestibule are two sacs distinct from each 

 other, and also from the lining membrane. 



One of these denominated Sacculus Sphericus, or Sacculus 

 Vestibuli, is situated partly in the Hemispherical Cavity of 

 the vestibule, and has no direct communication with any other 

 part. 



It contains a limpid fluid, and is said to have so much firm- 

 ness, that when opened with the point of a lancet, it will retain 

 its form. 



The other sac is situated partly in the depression called 

 Semi-elliptical, nearly opposite to the foramen ovale : it is so 

 transparent that it is sometimes seen with difficulty, and appears 

 like a bubble of air in a fluid. All the membranous semicircu- 

 lar canals, which are soon to be described, communicate with 

 it by each of their extremities: it has been called Alveus Com- 

 munis by Scarpa, and Utriculus by Soemmering. 



In the bony canals already described, are three Membra- 

 nous Semicircular Canals, which resemble them in form. 

 VOL. II. 40 



