803 



CHAPTER XII. 

 DISSECTION OF THE EAR. 



THE organ of hearing is made up of complex bodies, which are Subdivision 

 lodged in, and attached to the surface of, the temporal bone. It is 

 commonly divided into three parts, known as the external ear, the 

 middle ear, and the internal ear. Of these, the last is the essential 

 portion, containing the terminal expansion of the auditory nerve ; 

 and the others are to be regarded as accessory, serving to convey to 

 it the vibrations produced by the sonorous undulations of the air. 



EXTERNAL EAR. This includes the pinna or auricle and the Parts of 

 auditory canal : the former has been noticed at p. 569 et seq., and l 

 the latter remains to be described. 



The EXTERNAL AUDITORY CANAL (meatus auditorius externus ; Auditory 

 fig. 295) is the passage which leads from the pinna towards the canal: 

 tympanic cavity (a part of the middle ear), from which it is separated 

 in the recent state by the tympanic membrane. 



Dissection. To obtain a view of this canal, a recent temporal how to 

 bone is to be taken, to which the cartilaginous pinna remains vS^of^ 

 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 at the inner end which gives 

 support to the thin membrana tympani, is to be cut away with a 

 pair of bone- forceps. 



The canal is about one inch and a quarter in length, and is length ; 

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

 forwards somewhat obliquely, and describes a slight vertical curve direction ; 

 with the convexity upwards. In shape it is rather flattened from size and 

 before backwards ; and it is narrowest in the osseous portion. The sha P e ' 

 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 cartilagi- 

 length, and is formed chiefly by the pinna of the outer ear, which uous part 

 is attached to the margin of the osseous meatus ; but at the upper is deficient 

 and posterior aspect the cartilage is deficient, and the tube is closed above ' 

 by fibrous tissue. One or two fissures (fissures of Santorini) cross 

 the cartilage (p. 571). 



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

 adult, and is slightly constricted about the middle. Its outer part> 

 extremity is dilated, and the posterior edge projects farther than outer end 

 the anterior ; the greater portion of the margin is rough, and gives 



3 P 2 



