Fig. 21. Horizontal Section of the Head: Organ of Hearing and 



Surrounding Parts. 



Pig. 22. Tympanic Cavity and Surrounding Parts. 



X 2V2. 



Part of a frozen section from a series of Horizontal sections through the Head. 



The two curves described by the External Auditory Canal in the Horizontal 

 plane are well shewn : a 3rd curve is recognisable in the Vertical plane, so that 

 it is not possible to distinctly see the Tympanic Membrane without traction upon 

 the Cartilaginous portion of the Ear. 



From the practical point of view the relations which the Auditor}' Appa- 

 ratus bears to neighbouring structures and especially to the Lateral Sinus and 

 Dura Mater are most important. 



The Dura Mater which covers the posterior aspect of the Petrous Bone 

 is only separated from the numerous air-cells by a thin lamina of bone, so that 

 pus within these cells may easily give rise to a subdural abscess. The Lateral 

 Sinus bears a similar relation to the Mastoid Process so that injury to it may 

 result during operations upon the Mastoid; pus within the Mastoid Cells may 

 extend to the Lateral Sinus and produce Thrombophlebitis. 



Around the Internal Carotid Artery which incompleteh' fills the Carotid 

 Canal, are \^enous Spaces, continuations of the Cavernous Sinus. The Facial Nerve 

 is seen cut across in its vertical course to the Stylomastoid Foramen. Tn the 

 Facial Canal accompan3'ing the Facial Nerve (Fig. 22) are shewn the Stylomastoid 

 Artery with its 2 Venae Comites. In Fig. 22 are further shewn the structures 

 in the Tympanic Cavit}* : Ear-ossicles, Chorda T3'mpani and Tympanic Nerve. The 

 Eustachian Tube, extending obliquely forwards, inwards and downwards from 

 the Tympanic Cavity to open into the Pharynx has been divided obliquely so that 

 it appears larger than if it had been cut transversely to its axis. Between the 

 Condyle of the Lower Jaw and the External Auditory Meatus there is only a very 

 thin la)'er of bone. The Articular Fibrocartilage is seen almost completely 

 surrounding the Condyle. Internal to the External Pterygoid Muscle is seen the 

 3rd Division of the Vth Nerve directly after its emergence through the Foramen 

 Ovale; behind and to its outer side is the Middle Meningeal Artery. 



The Lymphoid Tissue beneath the Pharyngeal ]\Iucous Membrane — the 

 Pharyngeal Tonsil — which appears very large in a horizontal section deserves 

 notice. This has been cut obliquely because the upper wall of the Pharynx is 

 not horizontal but directed obliquely backwards and downwards. 



