370 .THE SPECIAL SENSES. 



The Projection of the Auditory Sensations. Auditory sen- 

 sations are projected to the exterior and, indeed, to the supposed 

 origin of the sound. The projection, however, is nothing like so 

 perfect as in the case of visual stimuli. Our judgments of the dis- 

 tance and direction of sounds are manifestly less exact than in the 

 case of objects seen by the eye. As an example, one may refer to 

 the difficulty of locating exactly such sounds as the note of a cricket. 

 In the ear the sensitive elements in the cochlea are not arranged so 

 that sounds coming from different directions can affect different 

 nerve fibers. All sound stimuli come to this part of the ear by one 

 path, namely, the tympanic membrane and its accessory struc- 

 tures. In judging the direction and distance of sounds we must 

 rely, therefore, upon the relative distinctness of the sounds in the 

 two ears, the variations in distinctness observed by varying the 

 position of the head, the accessory information obtained from 

 vision, etc. The general sensibility of the tympanic membrane 

 also plays a part. When a vibrating body a tuning-fork, for 

 example is held between the teeth, the vibrations are transmitted 

 to the internal ear in part at least through the bones of the head, 

 and the sound in this case is referred or projected into the head 

 itself instead of to the tuning-fork. So that in hearing by the usual 

 method the sensations of the vibrating tympanic membrane must 

 form part of the data by means of which we project the sensation 

 to the exterior. 



The Sensory Epithelium of the Cochlea. The fibers of the 

 cochlear branch of the auditory nerve arise in the nerve cells of the 

 spiral ganglion situated in the central pillar, the modiolus, of the 

 cochlea. This ganglion resembles in structure the posterior root 

 ganglion of the spinal nerves. Each cell is bipolar, sending one 

 fiber toward the brain in the acoustic nerve, and one fiber to end in 

 terminal arborizations around the sensory cells or hair cells of the 

 organ of Corti in the cochlea. We have every reason to believe, 

 therefore, that these hair cells form the apparatus which is affected 

 by sound and by means of which nerve impulses are generated 

 and transmitted to the acoustic fibers. The general arrangement 

 and the relations of these cells are indicated in Fig. 165. They 

 consist of short more or less cylindrical cells (E, 6, 6', 6", Fig. 165), 

 whose lower portion does not reach to the basilar membrane, but 

 is supported by the intervening Deiters cells. The upper ends of 

 the cells project through the openings in the reticulate membrane 

 and end in a number according to Retzius,* about twenty short, 

 stiff hairs. The hair cells are arranged in four to six rows, one 



* The most complete details of the structure of the ear will be found in 

 the great work of Retzius, "Das Gehororgan der Wirbelthiere," vol. ii, 1884, 

 Stockholm. 



