396 SENSITIVE SYSTEM. 



the inner, the annular cartiliio;e, the outer part. It is lined by 

 an extension of the skin from the concha, and is plentifully fur- 

 nished with sebaceous glands*. 



The Inlernal Ear. 



This comprehends two parts— the Tj/mpanum and the Labj/- 

 rinth. 



Tympanum. 



The tympanum consists of an irregular cavity, situated within 

 the petrous portion of the temporal bone, having the meatus au- 

 ditorius externus on its outer side, the labyrinth on its inner. 

 The cavity is lined by a very delicate mucous membrane, though 

 it contains no fluid; and exhibits several foramina or apertures. 

 At one part it has a communication, through the Eustachian 

 tube, with the external air ; but the entrance into it through the 

 meatus auditorius is closed by the membrana tympani. This 

 semi-transparent membrane is stretched tightly across the termi- 

 nation of the meatus ; not, however, after the manner of the 

 parchment of a drum, but funnel-like, a shape into which it is 

 drawn from the attachment to its centre of a lengthened process 

 belonging to one of the little bones contained within the cavity. 

 Two layers enter into its composition — an outer of skin, an inner 

 of lining membrane ; which last exhibits so high a state of vas- 

 cularity as to have been assimilated to the irisf. 



FetiestrcB. — The side of the cavity facing the meatus externus 

 exhibits a very irregular aspect, and is perforated by two aper- 

 tures — the fenestra ova/is and the fenestra rotunda. Between 

 these two perforations is an osseous eminence called the tubercle. 



The fenestra ovalis (also known ^?, i\\Q foramen ovale) 

 is an aperture bearing an approach to the fissure of an oval, 

 looking from the cavity of the tympanum into the labyrinth. 



The fenestra rotunda (also called the foramen rotun- 

 dnm) is likewise an ovoid opening, situated by the side of the 

 tubercle, and communicating (not with the labyrinth, as in the 

 case of the former opening, but) with the scalce of the cochlea. 



Bones. — Arranged along the interval between, and connected 



* In the foetus a thick whitish substance is secreted by these glands, 

 which answers the purpose of defending- the parts against any acrimony in 

 the liquor amnii. After birth, this secretion is discharged along with the 

 subsequent issue of waxy matter. 



t In the foetus, the membrana tympani (and, indeed, the tympanum alto- 

 gether) is very superficially placed ; for that which forms the osseous part 

 of the meatus externus in the adult is a mere bony ring at this period, 

 across which the membrane is extended. This is beautifully illustrated in 

 the cranium of a foetal puppy now bcf<tre the writer. 



