172 PHYSIOLOGY 



As sound is communicated by vibration, everything that does vibrate 

 is either capable of producing sound or of increasing it ; and perhaps air 

 has the least power of vibration of any substance or modification of 

 matter we are acquainted with : and, from experiment, water has been 

 found to be a much better vibrator than air. 



As this is the case, it would from thence appear, that an ear destined 

 to hear in water need not be so nicely constructed as one in an animal 

 whose way of life confined it to live in air ; and accordingly we find 

 them very different. 



The vibrations of the medium of sound in many animals are in- 

 creased before they reach the organ of sense by outworks, called the 

 external ear ; but this is not universal, belonging only to some of those 

 whose ears are adapted to the vibration of air, and even in them it 

 varies considerably in the different animals that have it : besides this, 

 there are other increasers of vibrations, such as membranes stretching 

 across the cavity, and other apparatuses besides. 



The most simple construction of the organ of sound in any of the 

 animals that I am yet acquainted with is that in fish. It is composed 

 of three canals, describing nearly a circle each, and so placed as to 

 make a triangle. Some of these communicate with one another at their 

 ends, others not. They all open into one cavity common to the whole. 



These canals, in this class of animals, are thin and transparent, and 

 of a cartilaginous substance, pretty regular in size through the whole, 

 excepting at or near to their unions, where they swell immediately into 

 round cavities. They are placed in the bones or cartilages of the skull 

 or head, and in canals or passages in these parts by much too wide for 

 them, and are supported in these passages by a very fine cellular mem- 

 brane. In many they project into the cavity of the skull. They appear 

 to have no external communication whatever 1 . 



The cavity formed by the union of the whole is pretty large ; in it 

 there is a bone of a particular shape in some, while it appears to be a 

 chalky substance in otbers, as in the skate, or ray tribe' 2 ; in all it is 

 perfectly detached : it is very large in all the cod tribe. Besides the 

 bone there is water, or a fluid, in the cavity. 



The nerves are very distinct in this order of ears : it appears that 

 they do not enter the cavity of these canals and spread upon their inner 

 surfaces, as is generally supposed to be the case in the human ear, but 

 seem to be attached to the external surface only, on which they spread 

 so as to enclose a little more of the canal. 



The next class of animals above fish is the Tricoilia. Their organ 

 of hearing becomes a little more complicated, having a greater variety 



i [Hunt, Preps. Nos. 1560-1568.] 2 [lb. Nos. 1569-1574] 



