832 THE ORGANS OF SENSE. 



year of life, so as to surround a foramen (foramen of Huschke) in the floor of the meatus ; this 

 foramen is usually closed by the fifth year, but persists until adult life in some 19 per cent 

 of skulls (Biirkner). 



The lumen of the meatus in the new-born child is extremely small : its outer part is funnel- 

 shaped ; its inner a mere slit, bounded below by the fibrous tympanic plate and above by the 

 obliquely placed membrana tympani. 



The skin which envelops the auricula lines the entire meatus, and covers also 

 the outer surface of the membrana tympani. It is thick in the pars cartilaginea, 

 and contains fine hairs and sebaceous glands, the latter extending for some 

 distance along the postero-superior wall of the pars ossea. The sudoriferous 

 glands are enlarged and of a brownish colour; they constitute the glandulse 

 cemminosae and secrete the ear wax or cerumen. 



Vascular and Nervous Supply of the Meatus. The external acoustic meatus receives its 

 blood-supply from the posterior auricular and superficial temporal arteries, and also from the 

 deep auricular branch of the internal maxillary artery, the last distributing some minute twigs 

 to the membrana tympani. The veins open into the external jugular and internal maxillary 

 veins, and also into the pterygoid plexus, while the lymph vessels have a similar mode of 

 termination to those of the auricle. Sensory nerves are supplied to the meatus by the 

 auriculo -temporal branch of the trigeminal and by the auricular branch of the vagus. 



CAVUM TYMPANI OB MIDDLE EAR. 



The tympanic cavity is a small air chamber in the temporal bone, between 

 the membrana tympani and the lateral wall of the internal ear or labyrinth (Figs. 

 707, 708). Lined with mucous membrane, it contains a chain of ossicles, malleus, 

 incus, and stapes, which reaches from its lateral to its medial wall, and transmits 

 the vibrations of the membrana tympani across the cavity to the internal ear. 

 Attached to the ossicles are several ligaments and two small muscles. 



The tympanic cavity consists of two portions : (1) The tympanum proper, or 

 atrium, lying medial to the membrana tympani ; and (2) the recessus epitympanicus, 

 or attic, lying above the level of the membrane and containing the greater part 

 of the incus and the upper half of the malleus. Including this recess, the 

 vertical and antero-posterior diameters of the tympanic cavity each measure 

 about 15 mm. The distance between its lateral and medial walls is about 6 

 mm. above and 4 mm. below, while at its central part, owing to the bulging 

 of the two walls towards the cavity, it measures only from 1-5 to 2 mm. 



The tympanic cavity is enclosed by six walls, tegmental, jugular, labyrinthic, 

 inastoid, carotid, and membranous. 



Paries Tegmentalis. The tegmental wall or roof (Fig. 709) is formed by a 

 thin plate of bone, the tegmen tympani, constituting a portion of the anterior surface 

 of the petrous part of the temporal. It extends backwards so as to cover in the 

 tympanic antrum, and forwards, to roof in the semicanal for the tensor tympaui 

 muscle. It separates the tympanic cavity and antrum from the middle fossa of the 

 cranial cavity, and may contain a few air-cells, whilst occasionally it is partly 

 deficient. In the child its lateral edge corresponds with the petro-squamous suture, 

 traces of which can generally be seen in the adult bone. 



Paries Jugularis. The jugular wall or floor is narrower than the tegmental 

 wall, and consists of a thin plate of bone which separates the tympanic cavity from 

 the fossa jugularis; anteriorly, it extends upwards and is continuous with the 

 posterior wall of the carotid canal. The inner orifice of the foramen for the 

 transmission of the tympanic nerve is seen near the junction of the jugular and 

 labyrinthic walls. 



Paries Labyrinthica. The labyrinthic or medial wall of the tympanic cavity 

 is formed by the lateral surface of the internal ear (Fig. 709). It presents (1) 

 a rounded eminence, the promontory, which is caused by the first coil of the cochlea, 

 and is grooved for the tympanic plexus of nerves. (2) An oval or somewhat 

 reniforrn opening, the fenestra vestibuli, which is situated above and behind the 

 promontory, with its long axis directed antero-posteriorly. It measures 3 mm. in 

 length and 1-5 mm. in width and, in the macerated bone, leads into the vestibule 

 of the osseous labyrinth, but is closed in the recent state by the base or foot-plate 



