?B ^'ATURAL THEOLOG-Y. 



structure of the ear a suitableness to receive impressions from 

 this species of action, and to propagate these impressions to 

 the brain. For of what does this structure consist ? An ex- 

 ternal ear, the concha,* calculated, like an ear-trumpet, to 

 catch and collect the pulses of which we have spoken ; in 

 large quadrupeds turning to the sound, and possessing a con 

 figuration as well as motion evidently fitted for the office 

 of a tube which leads into the head, lying at the root of this 

 outward ear, the folds and sinuses thereof tending and con- 



* Plate I., Fig. 5. a, the tube leading from the external ear ; hav- 

 ing little glands to secrete the wax, and hairs standing across it to 

 exclude msects without impeding the vibrations of the atmosphere ; 

 6, the membrane of the tympanum^ drawn into the form of a furmel by 

 the attachment of the malleus ; c, the chain of four bones lying in the 

 irregular cavity of the t}'mpanum, and comnaunicating the vibrations 

 of the membrane b to the fluid in the labyrinth; d, the eustachian 

 tube, which forms a communication between the throat and the tym- 

 panum, so as to. preserve an equilibrium of the air in the cavity of the 

 tympanmn and of the atmosphere : e, f, g, the labyrinth — consisting 

 of a central cavity, the vestibule g-, the three semicircular canals f, and 

 the cochlea J". 



Beginning from the left hand, (see also Fig. 6,) we have the mat 

 lexis or hammer, the first of the chain of bones ; we see its long han 

 die or process, which is attached to the membrane of the tympanum, 

 and moves as that vibrates ; its other end is enlarged, and has a groove 

 upon it which is articulated with the next bone. This second bone is 

 the incus or anvil, to the grooved surface of which the malleus is at- 

 tached. A long process extends from this bone, which has upon it 

 the OS orbiculare; to this third bone there is attached a fourth, the 

 stapes, which is in shape like a stirrup-iron. The base of this bone is 

 of an oval shape, and rests upon a membrane which closes the hole 

 leading into the labyrinth. Tliis hole is called the foramen ovale. 

 The plan of the cochlea shows that one of its spiral passages, begin- 

 ning in the vestibule e, winds round the pillar till it meets in a point 

 with another tube. If the eye follows this second spiral tube, it will 

 be found to lead, not into the vestibule, but into the irregular cavity 

 of the tympanum. Sounds striking against the membrane of the 

 tyxnpanimi, are propagated by means of the foiu small bones to the 

 water contained in the cavities of the labyrinth ; and by means of this 

 water the impression is conveyed to the extremities of the auditory 

 nerve and finally to the brain. 



