THE SKELETON 25 



probable function and modus operandi of these structures are dis- 

 cussed on p. 295. 



The lateral wall of the capsule is attached at three points to the 

 suspensorium (palato-quadrate), and the details of this attachment 

 will be considered in dealing with that part of the skull. The mesial 

 wall of the otic capsule, separating the labyrinth from the cavum 

 cranii, is perforated by five foramina, namely, the/or. perilymphaticum 

 (io.Tp\.)^t\icfor.endolymphaticum(Jo.t\.\ and three/or. acustka(f O.3.U.). 



The for. perilymphaticum (fo.pl.), through which the duct of the 

 same name passes into the cavum cranii from the labyrinth, is 

 situated close to the base of the capsule, towards the posterior end. 



The /or. endolymphaticum (fo.el.) serves a similar purpose for the 

 ductus endolymphaticus and lies a little anterior to the former — at 

 about the middle of the capsule — near the dorsal ^2S\.. 



The three /or. acustica (fo.au.) in the adult lie almost in the floor 

 of the capsule rather than in the mesial wall, owing to growth changes 

 in that region. They serve for the entry of the auditory nerve and 

 hence correspond to its three rami. They lie very close together and 

 immediately anterior to \kie for. perilymphaticum. 



The cavity of the capsule is not quite simple but is subdivided 

 by bridges which develop around the semicircular canals, the septa 

 semicircularia (Stadtmiiller). The septum semicircularium anterior^ pass- 

 ing from the antero-mesial to the dorsal wall, is very well developed, 

 while the sept, semicirc. later ale is considerably weaker and is not always 

 complete. The sept, semicirc. posterior is usually only faintly indicated 

 and never develops completely. 



The otic capsules are completely fused to the cranium and ample 

 traces remain of the parachordal plate. In the occipital region the 

 capsules are joined together by a broad hypochordal commissure (c.hyp. 

 com.), and, at their anterior ends, by a narrow bar of cartilage, the 

 crista retrosellaris (c.cr.rs.) (Stadtmiiller). Between the two bridges 

 the parachordals are represented only by slight ridges along the 

 ventro-mesial borders of the capsules. The auditory capsules are also 

 joined together dorsally by a narrow commissure at the posterior 

 end, the tectum synoticum (c.t.syn.). There are thus two fenestrae in 

 the floor of the primary cranium, an anterior and a posterior. The 

 fenestra basicranialis anterior (fen. be. a.) is the larger and is bounded 

 anteriorly by the planum internasale, laterally by the orbito-sphenoids 

 (representing the trabeculae), and posteriorly by the crista retrosel- 

 laris. The fenestra basicranialis posterior (fen. be. p.) lies between the 

 crista retrosellaris anteriorly and the hypochordal commissure pos- 

 teriorly, and is bounded laterally by the remains of the parachordal 



