444 NERVOUS SYSTEM. 



as to fill the whole cavity of the tympanum, and that without 

 ulceration. Sometimes, in such cases, the membrane of the 

 tympanum is ruptured, and the discharge finds its way out 

 through the rneatus externus, presenting itself under a purulent 

 form, as if an abscess had formed in the ear. 



SECT. III. OF THE LABYRINTH.* 



- 



The Labyrinth (Lalyrinthus) is placed on the inner side of 

 the tympanum, in the thickness of the petrous bone. Its ex- 

 terior parietes are bone, but internally there is a membranous 

 structure, having, in many respects, the same shape. It is got 

 at with great difficulty in the adult, owing to the compactness 

 of the petrous bone which envelops it; but in the foetus of the 

 full period, where it is almost as large as in the adult, the sur- 

 rounding bone is of a softer and more spongy texture, and may 

 be pared away with a pen-knife without much trouble. In the 

 latter case, the parietes of the bony labyrinth remain about 

 the thickness of an egg-shell, and have very much the same 

 degree of consistency and strength. 



The bony labyrinth consists of three portions: the Vestibulum, 

 the Semicircular Canals, and the Cochlea. 



The Vestibulum is the cavity to which the foramen ovale 

 leads; it, with the cochlea, occasions the protuberance into 

 the tympanum, known as the promontory. It is an irregular 

 rounded excavation, the surface of which is impressed by its 

 contents; thus, at the superior posterior and external part, 

 next to the semicircular canals, there is a superficial Fossa, 

 called, from its shape, Semi-Elliptica, and at its anterior and 

 inferior part, nearer the cochlea, another, called Fossa Hemi- 

 Spherica. These fossae are marked off from each other by a 

 ridge of bone, at the lower end of which there is a third fossa 

 between the other two, called by Scemmering, Cavitas sulci- 

 formis. 



There are seven orifices belonging to the vestibulum besides 

 the foramen ovale; five at its posterior part leading into the se- 



* Antonio Scarpa, Disquisit. de Auditu et Olfacto. 



