220 The Vertebrate Organ Systems 



sharp focus, another set will be focused either in front of or behind 

 the retina and will be fuzzy. This condition also is remedied by 

 proper lenses. 



The Auditory Mechanism. — For most terrestrial animals, the 

 ability to detect air-borne vibrations is of great survival value. Except 

 for the snakes, nearly all of the terrestrial vertebrates have some sort 

 of auditory mechanism. The snakes, although unable to detect air-borne 

 vibrations, are quite sensitive to ground vibrations ; thus it is unsafe 

 to assume that they cannot detect approaching footsteps. In mammals 

 and birds, the sense of hearing is extremely acute. 



semicircuTar 

 canals 



pharynx 'i^r 



Fig. 69. — A section through the ear of man. (By permission from General 

 Zoology by Storer, copyright, 1951, McGraw-Hill Book Company, Inc.) 



In mammals, the ear is composed of three principal regions : the 

 outer, the middle, and inner (Fig. 69). The outer ear is formed of a 

 flap of skin, the pinna, supported by elastic cartilages, and a passage- 

 way, the external auditory canal, for the passage of sound vibrations to 

 the middle ear. In man, the pinna is not of too much importance, for it 

 is quite small and cannot be moved although the muscles for moving it 

 are still to be found. Only a few talented persons can perform the 

 interesting trick of "wriggling" their ears. In an animal such as the 

 dog, however, the pinna can be moved in order to collect sound waves 

 more effectively. This undoubtedly also aids in localizing sounds, a 

 feat we can perform only by moving or tilting the head. 



The external auditory canal is a short passageway which ter- 

 minates at the eardrum or tympanic membrane. Within it are wax 



