THE NERVOUS SYSTEM 199 



shrimp oriented itself with reference to the resultant of the 

 lines of force of the magnet and the pull of gravity as it normally 

 would to gravity alone. From this it would appear that under 

 natural conditions the normal position of the animal is determined 

 by the stimulation of certain sensory hairs in the statocyst by 

 the weight of the statoliths. In the experiment, when these par- 

 ticular hairs are stimulated, so far as the animal is concerned, the 

 resulting orientation is normal even though the animal is tilted 

 at a considerable angle. 



Inner Ear. — Among vertebrates the inner ear serves both as 

 organ of hearing and as an organ of equilibration. It is present 

 in all vertebrates. Vertebrates higher than fishes also have a 

 middle ear, in addition to which mammals have an external ear. 

 The middle ear and external ear are part of the sound-conducting 

 apparatus employed in hearing. 



The inner ear arises as an invagination of the ectoderm in the 

 region of the embryonic hindbrain to form the otic sac, which 

 later becomes embedded in the cartilage or bone of the skull and 

 completely separated from the outside except in the case of 

 elasmobranchs. Two regions develop in the otic sac; (1) the 

 utriculus, bearing three semicircular canals, and (2) the sacculus, 

 which in fishes is a rounded sac with a short appendage, the 

 lagena (Fig. 129 A). In the frog the lagena is short, but near it 

 another outgrowth, the basilar papilla, is formed. In the higher 

 vertebrates the lagena and the basilar papilla of the sacculus 

 enlarge to form the scala media or cochlear duct which in mammals 

 becomes a spirally coiled tube (Fig. 129B). The utriculus with 

 its canals is concerned with equilibration, while the sacculus, the 

 lagena, and its derivatives are concerned with hearing. Fishes 

 are practically deaf so far as discrimination of pitch is concerned, 

 which may be correlated with the slight development of the 

 lagena. Goldfish respond to vibrations under water. By 

 removing parts of the inner ear it has been shown that the 

 utriculus is sensitive to vibration rates of 43 to 488 per second, 

 and the sacculolagena to rates of 688 to 2,752 per second. 



The utriculus and sacculus are connected by a sacculo-utricular 

 canal. The endolymphatic duct is a slender tube, closed at its 

 distal end, arising from the sacculo-utricular canal. In elasmo- 

 branchs it is lacking, but the cavity of the inner ear remains 

 connected with the outside by a slender duct, the invagination 



