THE NERVOUS SYSTEM 41 



Carefully shave off the cartilage of the elevations until 

 the canal within the cartilage is exposed. It contains a 

 curved, colorless tube, the anterior vertical semicircular 

 duct. Keep this duct intact and continue the dissection 

 until a posterior vertical semicircular duct is reached. Be- 

 low this is the thin-walled vestibule from which the semi- 

 circular ducts spring. A third duct, the horizontal semi- 

 circular duct, is below these and may be exposed by further 

 cutting away the cartilage, leaving the ducts and vestibule 

 in place. 



With the structures just exposed still in place identify 

 the parts of the internal ear. The ducts are semicircular 

 and each terminates in a sac, the ampulla. These are in 

 communication with the large dorsal utriculus which is 

 a part of the vestibule. The other ventral portion, the 

 saoculus, is located in a pit in the cartilage. The ampullae 

 are in connection with each other. Each is supplied with a 

 sensory area, the white crista, bearing the nerve. A white 

 mass of crystalline material or sand grains, the otolith, is 

 present in the utriculus. The endolymphatic duct also 

 enters the vestibule, but this entrance is difficult to locate 

 in the skate. 



The functions of the ear are those of hearing and equili- 

 bration. The canals in the cartilage are filled with peri- 

 lymph, and the ducts and vestibule with a fluid, endo- 

 lymph. Changes in pressure, either from sound waves or 

 the change in the position of the head, appear to excite 

 the sensory cells in the ampullae and vestibule. The am- 

 pullae and the vestibule control equilibration. The capacity 

 to detect sound waves seems to be limited to the vestibule. 



The above organs represent the inner ear of the higher 

 vertebrates. The middle and outer ears are absent in the 

 skate. 



Drawing 23. Make a drawing of the internal ear. 



