QUALITY OF AUDITORY PERCEPTIONS. 899 



(z y), which are inclined toward each other and meet above like the beams in 

 the roof of a house; but every two rods do not form an arch, as there are always 

 three inner to two outer rods. There are about 4500 outer rods. 



The cochlear duct becomes larger toward the apex of the cochlea, and the rods 

 also become longer. The inner ones are 30 fj. long in the first, and 34 n in the 

 upper turns; the outer rods respectively 47 // and 69 //. Likewise, the width of 

 the arches increases. The cylindrical hair-cells (cells of Corti), observed by 

 Corti, of which there are from 16,400 to 20,000, serve as the actual end- 

 organs of the cochlear nerve. There is one row of inner cells (i) which rest on 

 a layer of small granular cells (k) ; the outer cells (a a) number 12,000 in man, 

 and rest upon the basement membrane, in three or even in four rows. The 

 cells are directly connected by fibrous processes with the fibers of the basilar 

 membrane, so that each cell is connected with two or three fibers, and must, 

 therefore, vibrate in unison with the latter. Between the outer hair-cells there 

 are other cellular structures, which are regarded either as special cells (Deiter's 

 cells) , or merely as processes of the hair-cells. Following the outer cells of Deiter 

 come the cylindrical cells of Henle, which gradually pass into the ordinary epi- 

 thelium of the cochlear duct. 



The fibers of the cochlear nerve (N) emerge from the bony spiral lamina, 

 and, after passing through the intercalated ganglion-cells, (Fig. 325, I, G) end 

 by fine varicose fibrils on the hair-cells, with which their telodendrites are in 

 contact (Fig. 326). The bristles of the hair-cells consist in vertebrates of closely 

 massed fine fibrils. 



The arches of Corti and the hair-cells are covered by a special membrane 

 (o, reticular membrane), through openings in which project the upper extremities 

 of the hair-cells with the hairs. This membrane consists of cement-substance 

 holding these parts together. Mention should be made finally of the soft mem- 

 brane of Corti, which is comparatively thick, and extends from above outward 

 over the organ of Corti. Waldeyer regards this as a damping apparatus for the 

 organ of Corti. 



The fluid within the labyrinth also is under a constant pressure the 

 intralabyrin thine pressure. Every diminution in the pressure of the 

 air in the middle ear is accompanied by a temporary diminution in the 

 intralabyrin thine pressure, while, conversely, every increase in air- 

 pressure is accompanied by an increase in the intralabyrin thine pressure. 



The perilymph of the internal ear flows chiefly through the aqueduct 

 of the cochlea within the jugular foramen into the peripheral lymphatic 

 system, which also takes up the cerebrospinal fluid of the subarachnoid 

 space, while a small portion passes through the internal auditory meatus 

 to the subdural space. 



QUALITY OF AUDITORY PERCEPTIONS. 

 PERCEPTION OF THE PITCH AND INTENSITY OF TONES. 



Every normal ear is able to recognize musical tones and noises as 

 such, and to distinguish between them. Physical experiments have 

 proved that musical tones are produced when a vibrating, elastic body 

 executes a periodic movement, that is a movement that is exactly 

 reproduced at equal intervals of time, as in the vibration of a twanged 

 cord. A noise is produced when the vibrating object executes nonperi- 

 odic movements, that is, when unequal movements occur at equal time- 

 intervals. This is readily proved by means of the siren. If there be 

 on the circular disc of this instrument a number of holes, for example 

 forty, arranged in a circle and placed exactly the same distance from 

 each other, and if, on rotating the disc, a current of air is blown against 

 it, the air will be alternately rarefied and compressed exactly 40 times 

 with every revolution, and every two rarefactions and condensations 

 will be separated from each other by an equal interval of time. This 



