STRUCTURE OF THE VERTEBRATES 311 



canal. The ampullae are characterized by large sensory areas 

 with unusually long sensory processes on the cells. 



The ear of the dogfish functions in the same way as do those 

 of the cyclostomes. The endolymphatic fluid contains minute 

 crystals of calcium salts (which in some fish are collected into 

 a large otolith) and as the animal changes position the crystals 

 are washed against the sensory hairs. This gives the impression 

 of falling in that direction. The three canals of the typical 

 vertebrate give a complete record of movement in any direction; 

 and the dizziness which accompanies a whirling motion is due 

 to centrifugal force throwing the fluid away from the center, 

 and the impression made upon the brliin of falling in all di- 

 rections at once. 



The amphibia develop a cochlea, a coiled outgrowth from 

 the sacculus, which receives sound waves. The cochlea has its 

 beginning in the fish where a small lagena, or pouch, projects 

 from the ventral posterior portion of the sacculus. With the 

 progressive growth of the cochlea the powers of hearing become 

 greatly increased. In the mammal the cochlea is so highly coiled 

 that it becomes spiralled, and contains the specialized organ of 

 Corti. The function of this organ is speculative, for the birds, 

 which lack the structure, perceive tones and pitch. 



The inner ear is closely connected with the eighth, auditory, 

 nerve which spreads over it and enlarges into ganglionated 

 areas. The nerve is the essential organ of hearing, conveying the 

 impressions to the brain, and the associated structures are the 

 specialized receptors. 



Middle Ear. The 77iiddle ear first becomes a definite structure 

 in the anuran amphibia. It will be recalled that the first gill 

 pouch develops into a spiracle in elasmobranchs and crossop- 

 terygian fish. In the anura and amniotes the pouch does not 

 break through to the outside, but enlarges distally to form a 

 middle ear cavity, the proximal portion remaining as the Eu- 

 stachian tube. The external surface is covered by the tympanic 

 membrane, a fusion of the branchial pouch and the body cov- 

 ering. 



In the anura, reptiles, and birds, only the columella, or stapes, 

 is present, the embryonic hyomandibular having been taken 

 into the cavity. When the jaw articulation between the articular 



