COORDINATION 



403 



glasses or of contact lenses which fit tightly 

 against the eyeball. If the cornea clouds 

 over it can be replaced by one from a nor- 

 mal eye, restoring the vision to normal. This 

 is an extremely delicate operation but one 

 that is becoming more and more common. 

 People often "will" their corneas to others 

 at the time of death; this custom is preva- 

 lent enough now that cornea "banks" have 

 been established. 



Hearing 



Hearing is another sense that records 

 stimuli coming from a distance. This is a 

 convenient supplement to vision in that 

 sound travels around corners and in the 

 dark, two very important adjuncts to the 

 problem of orientation. Nearly all groups 

 of animals have some means of receiving 

 water or air vibrations (Fig. 16-8). Under- 

 water forms such as fish receive vibrations 

 in the water and can, therefore, hear, al- 

 though they possess none of the apparatus 

 found in higher vertebrates that is used to 

 receive and amplify sound. They do possess 

 the inner ear structures that are essential 

 for hearing. Apparently they can receive 

 only vibrations set up in their own bodies. 

 Air breathers, both vertebrate and inverte- 

 brate, have not only solved the problem of 

 receiving air vibrations but also of setting 

 up such vibrations. The katydid, for exam- 

 ple, produces its chirp by rubbing its wings 

 together, and the sound waves are received 

 by special "ears" located on the tibias of 

 the front legs. This form of communication 

 serves in bringing the sexes together in 

 mating and may also be important in noti- 

 fying other members of the species of fer- 

 tile food sources. 



When the vertebrates evolved onto land 

 they, too, started to solve the problem of 

 sound making and sound receiving in air, 

 although fish not only received sounds in 

 water but some even made sounds. The 

 frog has a crude ear and emits a very simple 

 sound. It is interesting to note that in pro- 

 viding for this change old structures, the 



gill arches, which now served no special 

 function, were employed in making the new 

 organs. For example, the tips of the jaws 

 and the hyoid arch which evolved earlier 

 from gill arches became the ear bones, and 

 some of the remaining gill arches became 

 the larynx (voice box) as well as other 

 elements of the upper respiratory system 

 (Fig. 25-11). In man, the production of 

 sound has probably been carried to its 

 greatest perfection, birds excepted, al- 

 though the sound receiving apparatus of 

 some other vertebrates, such as the dog, 

 has a greater range than the human ear. 



Intimately associated with and usually 

 considered as a part of the ear in the ver- 

 tebrates is another organ that is physiologi- 

 cally quite separate from the sense of hear- 

 ing. This is the organ of equilibrium, which 

 is a receptor for changes in conditions of 

 balance and rotation. In the invertebrates 

 these two organs are quite unrelated. Or- 

 gans of equilibrium are found in many 

 invertebrates, from the jellyfish to the cray- 

 fish. 



Among the lower vertebrates such as the 

 sharks and skates both the hearing organ 

 and the organ of balance are connected to 

 the brain through the eighth cranial or audi- 

 tory nerve. It is generally believed that 

 hearing came later in evolution and merely 

 took over a part of the eighth cranial nerve 

 which had initially functioned only in con- 

 veying impulses from the organ of equi- 

 librium. A discussion of the organ of equi- 

 librium will be found in a later section 

 (p. 408). 



The human ear is composed of three por- 

 tions, the outer, or pinna, the middle, which 

 connects with the throat by means of the 

 eustachian tube, and the inner, where the 

 cochlea (sound receptor) and semicircular 

 canals are located (Fig. 16-9). The outer 

 ear is merely a skin-covered, cartilaginous 

 projection from the head, designed to catch 

 and concentrate sound waves. In some ani- 

 mals, such as the mule and rabbit, this can 

 be moved in several directions to help de- 



