506 THE SPECIAL SENSES. 



partly bony ; which penetrates the lateral part of the temporal bone 

 in a nearly horizontal and transverse direction. In the human 

 subject, this canal is a little over one inch in length, and is lined 

 by a continuation of the external integument. The integument 

 toward its outer portion is beset with small hairs, and provided 

 with ceruminous glands which supply a secretion of a waxy or 

 resinous consistency. By these means the passage is protected 

 from the accidental ingress of various foreign bodies. 



Secondly, at the bottom of the external meatus the auditory pas- 

 sage is closed by a thin fibrous membrane, stretched across its cavity, 

 called the membrana tympani. Upon this membrane are received the 

 sonorous vibrations which have been collected by the external ear 

 and conducted inward by the external auditory meatus. Behind 

 the membrana tympani is the cavity of the middle ear, or the cavity 

 of the tympanum. This cavity communicates posteriorly with the 

 mastoid cells, and anteriorly with the pharynx, by a narrow passage, 

 lined with ciliated epithelium, and running downward, forward and 

 inward, called the Eustachian tube. A chain of small bones, the 

 malleus, incus, and stapes, is stretched across the cavity of the 

 tympanum, and forms a communication between the membrana 

 tympani on the outside, and the membrane closing the foramen 

 ovale in the petrous portion of the temporal bone. All the vibra- 

 tions, accordingly, which are received by the membrana tympani, 

 are transmitted by the chain of bones to the membrane of the 

 foramen ovale. The tension of the membranes is regulated by two 

 small muscles, the tensor tympani and stapedius muscles, which arise 

 from the bony parts in the neighborhood, and are inserted respect- 

 ively into the neck of the malleus and the head of the stapes, and 

 which draw these bones forward and backward upon their articu- 

 lations. 



Thirdly, behind the membrane of the foramen ovale lies the 

 labyrinth, or internal ear. This consists of a complicated cavity, 

 excavated in the petrous portion of the temporal bone, and com- 

 prising an ovoid central portion, the vestibule, a double spiral canal, 

 the cochlea, and three semicircular canals, all communicating by 

 means of the common vestibule. All parts of this cavity contain 

 a watery fluid termed the perilymph. The vestibule and semi- 

 circular canals also contain closed membranous sacs, suspended in 

 the fluid of the perilymph, which reproduce exactly the form of 

 the bony cavities themselves, and communicate with each other in 

 a similar way. These sacs are filled with another watery fluid, 



