OF HEARING. 173 



of parts annexed to it. They have the three semicircular canals as in 

 fish, but they are smaller and not so long. They lie in the bones of 

 the head, where there are very wide passages for them : they unite 

 into one common cavity, which has a chalk in it, as in the skate, &c. 



The additional parts in this class of animals. — From this hall, or 

 common cavity, passes outwards to the external surface a long small 

 bone, which is broad at its inner end or base, where it makes a part of 

 the hall : its outer end is attached to a membrane in most of this tribe ; 

 but to a cartilage in the turtle 1 , which is of an oblong figure, convex 

 externally and concave internally : this membrane is also convex on one 

 side and concave on the other, in the same position as the cartilage. 

 In most it is nearly in a line with the common surface of the body, as 

 in the lizard 2 , toad, frog 3 , &c. ; but it is placed somewhat deeper in the 

 crocodile 4 , which has something similar to an external ear; and it is 

 covered in some by the common integuments or scales, as in the turtle. 

 This cavity has an opening into the mouth, which is very probably no 

 more than a duct. 



The next class of animals above the Tricoilia is the birds. Although 

 their ear is not much more complicated than that of the Tricoilia, yet it 

 differs from it in some degrees. There is a neatness and precision in 

 the structure that is not to be found in the Tricoilia. The semicircular 

 canals in the bone are small and regular, and appear to answer the 

 purpose of these canals. If there are also the membranous canals, 

 then they are to be considered here as only linings to the bony. The 

 hall is smaller than in the former. 



The passage between the hall and membrane is enlarged and extended 

 into the meditullium or cells of the bones of the head, and much more 

 in some birds than in others. 



The membrane of the ear is not so superficial, so that there is a 

 canal, or a continuation of the same canal, beyond this membrane, 

 leading to the external surface, which terminates in particular forms in 

 different birds, which may be called an external ear, passage, or focal 

 [cavity 5 ]. 



The communication between the hall and membrane by means of 

 bone is similar to the former [Tricoilia, viz. by a single bone]. 



There is a passage from the ear into the mouth [Eustachian tube]. 



The next class of animals above the bird is that commonly called 

 quadruped. Their ear is much more complicated than any of the 

 former, having actually more parts. 



1 [Hunt. Preps. Nos. 1578-1580.] 2 [lb. No. 1576.1 



3 [lb. No.1575.] 4 [lb. No. 1577.] 



s [lb. No. 1581.] 



