782 THE SENSE OF EQUILIBRIUM 



it was placed at this time in the vicinity of finely pulverized iron, it used 

 this material instead of the sand, with equally beneficial results. Inas- 

 much as its otostatic cavities are situated at the base of the antennae 

 in free communication with the outside, the gravity of the iron could 

 be varied by means of magnets. Whenever this was done, the animal 

 immediately displayed pronounced disturbances in its movements, lead- 

 ing to a loss of its proper position in space. Very similar disorders have 

 been observed by Prentiss 1 in the larvae of lobsters which had been pre- 

 vented from obtaining a temporary substitute for their statolithic ma- 

 terial by placing them in filtered sea water. Streeter, 2 moreover, has 

 shown that tadpoles do not acquire the power of equilibration until the 

 sixth day after fertilization, i.e., not until the auditory vesicles have 

 made their appearance. The destruction of one of these organs gives 

 rise to disturbances in its equilibrium which may be rendered even more 

 pronounced by the removal of both. 



The Utricle and Saccule. While we have seen that the otolithic 

 cavity of the invertebrates is not an organ of hearing, it cannot be 

 denied that it serves as the precursor of the organ of hearing of the 

 higher vertebrates. The auditory sac, arising as a depression in the 

 epiblast near the hindbrain, becomes separated from the main tube, 

 but does not enter into direct communication with the outside. It 

 gradually develops into the variegated membranous labyrinth, consist- 

 ing eventually of the central canal of the cochlea, the saccule, utricle, 

 and 'the semicircular canals. In the lower vertebrates the cochlea 

 is absent, the first indication if it being presented by the cysticula of 

 the bony fish. Kreidl, 3 however, believes that this organ is still 

 too rudimentary to react to sounds; instead, he supposes the latter to 

 be received by the cutaneous sense-organs in consequence of vibrations 

 set up in the surrounding water. It should be noted, however, that 

 the fish are in possession of a statolithic sac to which one or more 

 semicircular canals are attached. The development of the latter 

 immediately suggests that these animals are also equipped with a 

 dynamic sense of equilibrium. 



Beginning with the terrestrial animals, the cochlea develops more 

 rapidly, it being present in an elementary form in the amphibia and 

 reptilia and in its more complete spiral form in birds. In the latter, 

 the central canal of the cochlea is united with the saccule as well as with 

 the other endolymphatic spaces. The labyrinth attains a structure 

 comparable to that of man, only in the higher mammals. Situated 

 directly within the osseous vestibule, we have two vesicular enlarge- 

 ments, namely, the saccule and utricle. The former attains a length 

 of about 3 mm. and a width of 2 mm. It is placed very close to the 

 orifice of the scala vestibuli of the cochlea. In the direction of the 



1 Bull. Mus. of Comp. Zoology, Harvard Univ., xxxvi, 1901. 



2 Jour, of Exp. Zoology, iii, 1906, 543. 



3 Pfliiger's Archiv, Ixiii, 1896, 581. The contrary view is held by Zenneck, 

 Pflttger's Archiv, xcv, 1903, 346. 



