THE NATURE OF SOUND. 399 



blance to the keys of a piano. Between the grooved margin of the 

 lamina spiralis, and the first set of the rods of Corti, is a cavity, and 

 between the second set and the external wall of the cochlea, is another 

 cavity, each containing nucleated cells of large size, named the cells 

 of Claudius. The zona denticulata becomes gradually narrower, from 

 the base to the summit of the cochlea ; and so, in fact, must, more or 

 less, all the structures of the membranous part of the spiral lamina. 

 The outer margin of the membranous part of the spiral partition of the 

 cochlea, has been described by some, as being composed of involuntary 

 muscular fibres, and has received the name of the cochlear muscle. 

 But this is not generally admitted ; its structure being regarded as of 

 the nature of delicate periosteum. 



The auditory or acoustic nerve, the portio mollis or soft portion of 

 the seventh cranial nerve (p. 251), is the special nerve for the sense 

 of hearing. In the petrous portion of the temporal bone within the 

 cranium, is a short canal or passage, known as the internal auditory 

 meatus, the bottom of which corresponds with the vestibule and the 

 base of the cochlea, and is perforated by numerous small openings, 

 whence it is named the cribriform plate. The auditory nerve, Fig. 77, 

 n, enters this meatus, and there divides into two branches, named the 

 cochlear and vestibular nerves, the funiculi of which pass through the 

 minute openings in the bottom of the meatus, into the labyrinth. The 

 nerves destined for the saccule, utricle, and membranous semi-circular 

 canals, are gathered into five or six bundles, invested and supported 

 by the lining membrane of the cavity. Two of these bundles pierce 

 the walls of the saccule and utricle, at the situations of the otolithes ; 

 here the fibres spread out, some radiating on the inner surface of the 

 walls of the cavities, others lying amongst the earthy particles, and 

 ending in free points. The remainder of these bundles are distributed 

 to the ampullae of the membranous semicircular canals, within which 

 they end in a manner not yet understood, certain fine hair-like pro- 

 cesses, here visible, being possibly the ends of the nerves, or else a 

 fine hair-like epithelium. The numerous filaments of the cochlear 

 nerve, ascend along small canals running up the modiolus ; they then 

 diverge laterally, in regular succession, along other little channels 

 formed in the bony part of the lamina spiralis, on the under surface of 

 the margin of which, that is, in the tympanic scala, they form a plexus, 

 which contains ganglionic nerve-cells. The branches from this plexus, 

 pass through the habenula perforata of the bony lamina spiralis, to 

 reach the scala media. The ultimate fine and free extremities of these 

 nerves, are said, by Kblliker, to end in the fluid of the scala media, 

 where they probably become connected with the rods of Corti ; some 

 are also supposed to pass amongst, or into, the cells of Claudius. 



Sound and its Propagation. 



Sound, as sound, has no existence in nature, and, indeed, cannot 

 exist independently of a sense of hearing. When sound is generated, 

 certain disturbances of state in elastic bodies occur, as the result of 



