NERVOUS SYSTEM 869 



sense of hearing is very incomplete without this structure, we must come 

 to the conclusion that the lower forms of life which do not possess such 

 an organ must either hear entirely different sounds from those that we 

 hear, or some other part of the structures they do have must do work 

 other than that done in man. A very complicated organ of Corti arises 

 within the scala media. Fine hair-like structures develop in the organ 

 of Corti which can only be worked out microscopically and with great 

 difficulty. There is a membrane extending out from the middle wall 

 over some of the hair cells in the organ of Corti, called the membrana 

 tectoria. Various functions have- been assigned to this membrane, one 

 of them being the ability to recognize pitch in sound. Birds, however, 

 have no organ of Corti at all, and the evidence is quite conclusive that 

 they can distinguish pitch. 



The Middle Ear or Tympanic Cavity. This first appears in the 

 Anura, that is, in the tailless amphibians such as the frog. We have 

 already seen from our dissection of the frog that a tympanic cavity con- 

 nects with the pharynx by a slender duct, the Eustachian or auditory 

 tube. Externally there was a tympanic membrane extending across the 

 fenestra ovale, through which sounds were transmitted to the inner ear. 

 The Eustachian tube is usually considered the homologue of the narrow 

 internal end of the spiracle in the dogfish. Frogs, birds, and reptiles 

 have a chain of ear bones consisting of a columella and stapes, while in 

 the mammals the incus and malleus replace the columella (Fig. 487). 



The External Ear. It will be remembered that in the frog the tym- 

 panic membrane lies on a level with the surface of the head. In higher 

 forms the tympanic membrane lies at the bottom of a canal called the 

 external auditory meatus. In most mammals, with the exception of 

 those that live in water such as the whales and seals, there is even an 

 external conch developed behind the meatus, so as to assist in collecting 

 the sound waves and directing them internally. In some of the birds, 

 feathers are arranged about the external meatus to function as does this 

 conch. 



It is to be remembered that the ear not only serves the purpose of 

 taking in and interpreting sounds, but that the semi-circular canals, lying 

 as they do in three dimensions of space and filled with the endolymph, 

 function somewhat similar to a carpenter's level, sending sensations to 

 the brain by which the animal recognizes the position of its own body 

 relative to its surrounding environment. We may therefore think of the 

 semi-circular canals as an organ of equilibration. 



THE NOSE 



The real sensory part of the smelling apparatus is always restricted 

 to one or two small patches of olfactory epithelium near the end of the 

 head. As the olfactory sac sinks beneath the surface of the ectoderm, 

 it remains connected by a pair of external openings called the nares. 



