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course, still further space for the expansion of 

 the auditory nerve. It is true these parts are not 

 common to all reptiles ; serpents, for instance, 

 having no tympanum although they have a 

 small bone, analogous to those which, in other 

 reptiles, are situated in this cavity, but which, in 

 serpents, is lost in the muscles of the jaws and 

 none but some of the highest orders of lizards, as 

 the crocodile, having a cochlea. The last named 

 animal, moreover, makes the first approach to 

 the well known appendage to the ear, technically 

 called the pinna ; being furnished with a kind of 

 external flap, with which it closes the auditory 

 apparatus at pleasure. It is in this way proba- 

 bly that the animal excludes too intense sounds 

 when under water ; but it appears that the greater 

 number of amphibious animals are capable of 

 adapting their auditory apparatus, at least parti- 

 ally, to the medium in which they are, by putting 

 all the parts upon the stretch, by means of the 

 muscles already spoken of, when in the air, so as 

 to qualify them to receive slighter impressions, 

 and by throwing them all into a state of relaxa- 

 tion when under water, so as to prevent them 

 from being stunned by more powerful ones. 



In birds at length we meet with constantly a 

 short canal, leading from the side of the head, 

 and meeting that coming from the pharynx, in 

 the tympanum. They have but one bone in this 

 cavity ; and the general structure of the parts of 

 their labyrinth is very similar to that of the higher 

 orders of reptiles. Birds in general want a pro- 



