CHAPTER IX 



THE EAR, THE EYE, THE SKIN, AND THE FOOT 



THE EAR 



The ear is the organ that has to do with the sense of hearing. 

 It is subdivided by anatomists into the external ear, the middle 

 ear, and the internal ear. 



The external ear comprises three cartilages, the largest of 

 which is the trumpet-shaped conchal cartilage which determines 

 the shape of the ear and is covered by thin skin. The annular 

 cartilage is bent to form a tube which is connected with the osse- 

 ous tube noticed on the temporal bone. Wax or cerumen is se- 

 creted by glands in this part of the ear. 



The middle ear is called the drum on account of the thin tym- 

 panic membrane like a drumhead which separates the tympanic 

 air-containing cavity from the external ear. The tympanic 

 membrane vibrates in response to the vibrations in the air which 

 come in contact with it. Across it a chain of three bones is 

 stretched. These bones transmit the vibrations of the tympanic 

 membrane to the inner ear. The bones are named the malleus, 

 from its resemblance to a hammer; the incus, which is shaped 

 like an anvil; the stapes, which is stirrup shaped. A connection 

 is made between the middle ear and the pharynx by means of 

 the eustachian tube to equalize the pressure of air upon the drum- 

 head. In. solipeds a diverticulum known as the guttural pouch 

 is found on the course of this tube. 



The internal ear is composed of soft structures, fluids and a 

 series of chambers in the substance of the temporal bone. In 

 it are found the ramifications of the auditory or eighth cranial 

 nerve, which has to do with the reception of sound-waves. 

 Although the actual organ of hearing is the cochlea, a bony 

 structure similar in shape to a snail's shell, the impulses must be 

 carried over nerve fibers to the brain for interpretation. 



The function of the ear is to record the sensations of sound. 

 When sound-waves reach the ear-drum they set it in motion. 

 In turn, the latter causes the chain of ossicles to vibrate, and the 



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