148 THE SKULL. [chap. 



looks like a large hole opening directly into the side of the 

 bulla. On looking into this hole, at a very short distance 

 (just beyond the tympanic ring), a wall of bone is seen quite 

 impeding the view, or the passage of any instrument, into 

 the greater part of the bulla. In the section (Fig. 53) it will 

 be seen that this wall is a septum (s), which rises from the 

 floor of the bulla, along its outer side, and divides it almost 

 completely into two distinct chambers ; one (oc), outer and 

 anterior, is the true tympanic chamber, and contains the 

 tympanic membrane and ossicula, and has at its anterior 

 extremity the opening of the Eustachian tube (e); while the 

 other (ic), internal and posterior, is a simple but much 

 larger cavity, having no aperture except a long but very 

 narrow fissure (*) left between the hinder part of the top 

 of the septum and the promontory of the periotic, which 

 fissure expands posteriorly, or rather at its outer end, into a 

 triangular space, placed just over the fenestra rotunda, so 

 that the opening of this fenestra is partly in the outer and 

 partly in the inner chamber of the bulla. . This chamber is 

 formed by a simple capsule of very thin but dense bone, 

 deficient only at a small oval space in the roof, where the 

 periotic projects into and fills up the gap, except such 

 portion of it as is left to form the aperture of communica- 

 tion with the outer chamber. 



Not only are these two chambers thus distinct, but they 

 are originally developed in a totally different manner. At 

 birth the only ossification in the whole structure is the in- 

 complete ring of bone supporting the membrana tympani, 

 and developed originally in fibrous tissue. Ossification 

 extends from this, so as to complete the outer chamber, 

 and the very limited lip of the. meatus auditorius externus. 

 The inner chamber is formed from a distinct piece of hya- 

 line cartilage, which at birth is a narrow slip, pointed at 



