No. I.] THE VERTEBRATE EAR. lyy 



sea-water. In many forms of Elasmobranchs the ear contains 

 scarcely any crystals, and not unfrequently sand grains are found 

 within the ear. The sea-water undoubtedly plays a consider- 

 able part in removing the otoliths that are found in all species, 

 by dissolving them. 



The mechanical forces active in the modelling of the ear are 

 for the most part the inherited tendencies of cell growth ac- 

 quired as legacy from the canal organs of the surface of the 

 body. The other forces are the pressures due to enlargement 

 of ear and the concomitant reduction of space in which it 

 has come to lie, the growth of the nerves into the ear, and the 



Cut 20. — Diagram to illustrate the relations of sense organs of the ear when the ear 

 vesicle is spread upon the surface of the head. 



establishment of the vascular supply. The cochlear tube begins 

 as a straight evagination, and curves soon after it comes in con- 

 tact with the body wall. Inherited tendencies suffice to bring 

 about the division of the primitive sensory area, the formation 

 of the ampullae, and the canals, except so far as the canals have 

 a more or less curved outline. The canals are curved for the 

 reason that they are formed as flattened pockets given off from 

 a curved surface, and by the fusion of the edges of the fold 

 along the whole mouth of the pocket, except at the two ends, 

 where the tube thus formed opens into the utricular cavity at 

 both ends. More clearly to understand that this is so, suppose 

 the auditory vesicle with canals well formed be again drawn to 



