392 



THE SKIN AND THE SENSE-OKGANS. 



by a bony ridge, which marks the position of the anterior semi- 

 circular canal. Posteriorly and medianly it is separated from the 

 posterior surface by a cartilaginous ridge, running from above, 

 downwards and outwards, which marks the position of the posterior 



Fig. 244. 



semicircular canal. The cartilaginous 

 hinder root of the processes squamosus 

 prootici arises in the cartilaginous ridge 

 just mentioned, and runs outwards and 

 forwards, covering the external semi- 

 circular canal. 



The lower half of the supero-external 

 surface (under the processus gquamosus) 

 is irregularly concave; immediately 

 under the proc. squamosus is a shallow 

 groove in the prootic ; it is continued 

 on the exoccipital to the jugular fora- 

 men. The remaining part of this sur- 

 face consists of a rounded cartilage, and 

 is part of the primordial-cranium car- 

 tilage between the prootic and the 

 exoccipital ; posteriorly it has a small 

 oval aperture, foe foramen ovale (fenestra 

 vestibulare), which with the groove is 

 Above the aperture the cartilage narrows 

 and is continued to the processes squamosns. 



(2) The posterior surface is directly continuous with the supero- 

 external surface, and is formed by the exoccipital. This surface lies 

 behind the ridge formed by the posterior semicircular canal, it is 

 concave, and has two small apertures, separated by a narrow, bony 

 process, and situated near the jugular foramen ; these are the 

 foramen rotundum (/K) and the aquaeductiis cochleae (ac.). 



(3) The anterior surface is altogether bony and formed by the 

 prootic (pr.}; the anterior surface of this part of the wall is continuous 

 with the surface of the cranium, and abuts on to the large tri- 

 geminal foramen (ft.}; below it is continuous with the under 

 surface of the cranium, below and externally it articulates with the 

 pterygoid. 



(4) The interne-inferior surface is as a whole hollowed in- 

 ternally, the upper part being pushed in towards the cranial cavity. 

 It is formed by the prootic anteriorly, by the exoccipital posteriorly, 

 and is completed by the cartilage lying between these bones. In 



J3S. 



Tin. 



Antero-posterior section through he 

 capsule of the right labyrinth of 

 Rana escuknta ; after Retzius. En- 

 larged five times. 

 ac. Aquaeductus cochleae. 

 av, Aquaeductus vestibuli. 

 cp. Posterior semicircular canal. 

 era. Canalis ranii anterioris acustici. 

 crp. Canalis rami posteriori* acustici. 



Fenestra rotunda. 



Fovea sacculi et cochleae. 



Trigeminal foramen. 



Cartilaginous suture. 



Exoccipital. 



Prootic. 



Parasphenoid. 



fr- 



fs<: 



fl. 



Jin. 



ol. 



pr. 



ps. 



covered by the columella. 



