184 THE HUMAN BODY. 



De Blainville compares the curves and the surface of the 

 pavilion of the ear to those of the head. According to this 

 naturalist the curve of the superior portion of the pavilion 

 corresponds to that of the cranium, and the free border of 

 the rim describes a curve parallel to that which marks the 

 temporal fossa. When the head is not prominent .in its 

 middle region, and the temporal fossae are slightly marked, " 

 the rim is absent; it is, on the contrary, broad and prominent 

 when the arch of the skull overhangs the temporal fossae. 

 The concha corresponds to the upper jaw, and is propor- 

 tional to it; and the prominence of the origin of the helix 

 represents that of the zygomatic arch; and lastly, the profile 

 of the lobe is like that of the upper jaw. It is remarkable 

 that this lobe exists only in man, and that man only has also 

 a prominent and angular chin. 



The auditory canal, which represents the tube of the acoustic 

 trumpet formed by the external ear, is cartilaginous next the 

 concha, and the remaining part is excavated in the petrous 

 or stony portion of the temporal bone. This canal is about 

 one and a fifth inch in length, and its entire disposition is such 

 that foreign bodies suspended in the air cannot pass with it 

 to the membrana tympani. It is bent near the concha in 

 such a manner that the air, in transmitting sound to the middle 

 ear, does not penetrate in right lines, thus protecting the 

 sensibility of the membrane. 



The middle ear. The membrana tympani (membrane of 

 the drum), of which the name indicates the function, is a 

 membranous partition stretched obliquely across the bottom 

 of the auditory canal, which it separates from the middle ear 

 or drum. This membrane is semi-transparent and very thin, 

 although it is composed of three layers; it vibrates under the 

 impression of the sonorous waves, and transmits the vibratory 

 movement to the little bones of the ear. Between the 

 membrana tympani and the internal ear is the drum or the 

 tympanum, a cavity hollowed out, like all those of the middle 

 and internal ear, in the petrous portion of the bone. Among 

 the details of its form and organization, we remark the 

 fenestra ovalis, which communicates with the vestibule, and 

 \\\zfenestra rotunda, which leads to the cochlea. The drum 



