THE TYMPANUM 



77 



The styloid process is ossified in cartilage from two centres, one of which appears at the 

 base in the tympano-hyal before birth. This soon joins with the temporal bone, and in the 

 second year a centre appears for the stylo-hyal, which, however, remains very small until pu- 

 bert^^ In the adult it usually becomes firmly united with the tympano-hj-al, but it may remain 

 permanently separate. 



THE TYMPANUM 



The tympanum (middle ear) includes a cavity [cavum tympani] of irregular 

 form in the temporal bone, situated over the jugular fossa, between the petrous 

 portion medially and the tympanic and squamous portions laterally. When fully 

 developed, it is completely surrounded by bone except where it communicates 

 with the external auditory meatus, and presents for examination six walls — 

 lateral, medial, posterior, anterior, superior (roof), and inferior (floor). The 

 lateral and medial walls are fiat, but the remainder are curved, so that they run 

 into adjoining surfaces, A\dthout their limits being sharply indicated. 



The roof or tegmen tympani [paries tegmentalis] is a translucent plate of bone, forming 

 part of the superior surface of the petrous portion and separating the tympanum from the 

 middle fossa of the skull. The floor [paries jugularis] is the plate of bone which forms the roof 

 of the jugular fossa. 



The medial wall [paries labyrinthica} is formed by the tympanic surface of the petrous 

 portion. In the angle between it and the roof is a horizontal ridge which extends backward as 

 far as the posterior wall and then turns downward in the angle between the medial and posterior 

 walls. This is the facial (Fallopian) canal, and is occupied by the facial nerve. The other 

 features of this surface — viz., the fenestra vestibuli, the fenestra cochleae, and the promontory 

 — have previously been described with the anterior surface of the petrous portion of the temporal 

 bone. 



The posterior wall [paries mastoidea] of the tympanum is also formed b}' the anterior 

 surface of the petrous portion. At the superior and lateral angle of this wall^an opening 



Fig. 97.^The Medial Wall of the Tympanum. 



Carotid canal ■*■- 



Tensor tympani 



Groove for Eustachian 



tube 



Levator veil palatini' 



Canal for small deep. 



petrosal nerve 



Stylo-pharyngeus 



Stylo-hyoid 



Stylo-glossus 



Lateral semicircular 



canal 

 Mastoid antrum 

 Facial canal 



Canal for chorda tympani 

 Stylo-mastoid foramen 



leads into the mastoid antrum. Immediately below this opening there is a small hollow cone, 

 the pyramidal eminence, the cavity of which is continuous with the descending limb of the 

 facial canal. The cavity is occupied by the stapedius and the tendon of the muscle emerges 

 at the apex. One or more bony spicules often connect the apex of the pyramid with the 

 promontory. 



The roof and floor converge toward the anterior extremity of the tympanum, which is, in 

 consequence, very low; it is occupied by two semicanals, the lower for the Eustachian tube, the 

 upper for the tensor tympani muscle. These channels are sometimes described together as the 

 canalis musculo-tubarius. In carefully prepared bones the upper semicanal is a small hori- 

 zontal hollow cone (anterior pyramid), 12 mm. in length; the apex is just in front of the fenestra 

 vestibuH, and is perforated to permit the passage of the tendon of the muscle. As a rule, the 

 thin walls of the canal are damaged, and represented merely by a thin ridge of bone. The 

 posterior portion of this ridge projects into the tympanum, and is known as the processus 

 cochleariformis. The thin septum between the semicanal for the tensor tympani and the 

 tube is pierced by a minute opening which transmits the small deep petrosal nerve. 



The lateral wall [paries membranaceaj is occupied mainly by the external auditory meatus. 

 This opening is closed in the recent state by the tympanic membrane. The rim of bone to 

 which the membrane is attached is incomplete above, and the defect is known as the tympanic 

 notch (notch of Rivinus). Anterior to this notch, in the angle between the squamous portion 

 and the tympanic plate, is the petro-tympanic (Glaserian) fissure, and the small passage 

 which transmits the chorda tympani nerve, known as the canal of Huguier. 



Up to this point the description of the middle ear conforms to that in general usage. But 

 Young and MiUigan have laid stress on the fact that the middle ear is really a cleft, named 

 by them th,e "middle-ear cleft," which intervenes between the periotic capsule, on the one hand, 

 and the squamo-zygomatic and tympanic elements of the temporal bone on the other. This 

 cleft, as development proceeds, gives rise to three cavities: — (1) the mastoid antrum; (2) 



