FISHES, 335 



Govery directed the attention of anatomists to the structure 

 of the organs of hearing, and Camper, Geoffroy, Hun- 

 ter *, and Vica d'Azyr, succeeded in pointing out the 

 nature of the different parts. Our illustrious countryman 

 Dr Monro, in his work on the structure and physiology of 

 fishes, contributed to enlarge our knowledge of the organs 

 of hearing, in this tribe, by numerous accurate dissections. 

 In the osseous fishes, no external ear has hitherto been 

 detected, and the same remark is applicable to those carti- 

 laginous fishes which have free branchiae. But in the car- 

 tilaginous fishes with fixed branchiae, small apertures have 

 been discovered leading to auditory organs. These were 

 first observed by Monro, in the skate and the angel shark. 

 In the former fish they occur in the back part of the occi- 

 put, near the joining of the head with tlie spine. They 

 are two in number, not larger than to admit the head of a 

 small pin ; and in large fish are. found at the distance of an 

 inch from each other. 



In fishes that have free branchise, the internal organs of 

 hearing are situated in the sides of the cavity of the cra- 

 nium, and fixed there by a cellular tissue, consisting of ves- 

 sels, and osseous or cartilaginous frsena. In the fishes with 

 fixed branchiae, these organs are inclosed in a particular ca- 

 vity, formed in the substance of the cranium. This cavity 

 is situate on the side and posterior part of that which con- 

 tains the brain, with which it does not communicate, except 

 by the holes that afford passages for the nerves. This sac 

 exhibits many differences as to size and form, in the dif- 

 ferent species. Besides the ordinary viscid fluid, there are 

 some small cretaceous bodies suspended by a beautiful 

 plexus of nerves. Tliese, in the osseous fishes, are three 



• Phil. Trans. 1782, p. 373. 



