174 PHYSIOLOGY 



In this class the semicircular canals are similar to the former 1 , but 

 we have passing from the hall another or fourth canal, which is coiled 

 upon and within itself, called cochlea 2 . 



The tympanum is extended some way into the bones of the head 3 ; 

 in some much more so than in others, as in the elephant, similar to 

 many birds. The membrane is more internal than in the former, which, 

 of course, makes the distance between that membrane and the external 

 surface still greater. It is concave externally, contrary to the fore- 

 going. The communication between the hall and membrane is by 

 three bones 4 instead of one. 



The passage from the membrane outwards is of considerable length ; 

 first in the bones, then continued further by means of a chain of carti- 

 lage, making a pipe, which wheu got to the external surface spreads in 

 most into various forms and length, called the external ear. But this 

 last part is not to be found in all : it is not in any of the whale kind, 

 perhaps because the water is sufficient of itself 5 ; nor is it of any size 

 in the seal kind, perhaps because they are intended to search after their 

 prey in the water, therefore not necessary. Nor are they to be found 

 in many animals whose life is principally led underground, such as the 

 mole ; and perhaps because the earth assists considerably in vibration. 



Query : Does the membrane of the ear increase the sound by increa- 

 sing the number of vibrations, or by increasing only the same vibration ? 

 or does it only communicate the first vibration in the air ? I should be 

 apt to suppose the first. 



All animals that have no external ear projecting from the head, as 

 birds, lizards, and, I believe, many of the amphibia and sleeping 

 animals, have their membrana tympani convex externally. How far 

 this is a substitute for an external ear is not easily determined ; but it 

 would appear to have some such effect, as it is so universal. This class 

 have no cochlea. Snakes and tortoises have no [obvious] external 

 passage to the ear. 



It is most probable that all animals which are capable of forming 

 sound have the organ of hearing, but not conversely ; for there are 

 many animals that are not capable of making sounds themselves that 

 have the organ of hearing, e. g., fish. 



In those animals which have external projecting ears and a consider- 



i [Hunt. Prep. No. 1603.] 2 [lb. No. 1599.] 



3 [lb. No. 1601.] 



4 [lb. No. 1602 : from this passage it would appear that Hunter considered the 

 os orbiculare as an epiphysis merely, as it has subsequently been regarded by several 

 anatomists. See Carlisle, Phil. Trans. 1805, p. 201.] 



s [Hunt. Preps. Nos. 1582-1598.] 



