670 DISSECTION OF THE EAR. 



CIIAPTEE XL 



DISSECTION OF THE EAR. 



THE organ of hearing is made up of complex parts, which are lodged 

 in, and attached to the surface of the temporal bone. 



The fundamental structure, as in the eyeball, is an expansion of a spe- 

 cial nerve over a membrane containing fluid. This delicate apparatus is 

 inclosed in bone for its protection ; and it is surrounded by certain acces- 

 sory bodies which convey to it the vibrations produced by the sonorous 

 undulations of the air. 



The auditory apparatus may be arranged into the parts outside, and 

 those within the substance of the temporal bone. 



In the EXTERNAL SET which may be first examined, are included the 

 pinna or auricle, and the auditory canal: the former has been noticed at 

 p. 45, and the latter is described below. 



The AUDITORY CANAL (fig. 238) (meatu.s auditorius externus} is the 

 passage which leads from the pinna to the cavity (tympanum) in the tem- 

 poral bone, and transmits inwards sonorous undulations of the air. 



Dissection. To obtain a view of this canal, a recent temporal bone is 

 to be taken, to which the cartilaginous pinna remains attached. After the 

 removal of the .soft parts, the squamous piece of the bone in front of the 

 Glaserian fissure is to be sawn off; and the front of the meatus, except a 

 ring internally which gives support to the thin membrana tyrnpani, is to 

 be cut away with a bone forceps. 



The canal (fig. 238) is about one inch and a quarter in length, and is 

 formed partly by bone, and partly by cartilage. It is directed forwards 

 somewhat obliquely. In shape it is rather flattened from before back- 

 wards; and it is narrowest in the osseous part. The outer extremity is 

 continuous with a hollow (concha) of the external ear, and the inner is 

 closed by the membrana tympani. 



The cartilaginous part (a) is largest. It is about half an inch in 

 length, and is formed chiefly by the pinna of the outer ear which is at- 

 tached to the margin of the meatus; but at the upper and posterior aspect 

 the cartilage is deficient, and the tube is closed by fibrous tissue. One or 

 two fissures (fissures of Santorini) cross the piece of cartilage. 



The osseous part (6) is about three-quarters of an inch long in the adult, 

 and is constricted sometimes about the middle. Its outer extremity is 

 dilated, and the posterior part projects farther than the anterior; the greater 

 portion of the margin is rough, and gives attachment to the cartilage of 

 the pinna. The inner end is smaller, and is marked in the dry bone by a 

 groove for the insertion of the membrane of the tympanum; it is so sloped 

 that the anterior wall juts beyond the posterior by about two lines. 



In the foetus the osseous part of the meatus is absent. After birth it 

 grows out of the osseous ring (tympanic bone) which supports the mem- 

 brana tyrnpani. 



Lining of the meatus. A prolongation of the integument lines the 



