No. I.] THE VERTEBRATE EAR. 265 



which were thus sympathetically affected, but in the fourth 

 edition of his work he substitutes the fibres of the membrana 

 basilaris, which, according to the measurements of Hensen and 

 C. Hasse, he held to be admirably suited for the work. The 

 membrane is composed of radial fibres in two or more sheets, 

 but the fibres do not extend through the whole breadth of the 

 basilar membrane. These sheets are covered above and below 

 by cellular and connective tissue of no inconsiderable mass as 

 compared with the mass of the fibres of the membrane, but 

 which most writers, following Helmholtz, have ignored as not 

 affecting the result. Corti's arches are now considered by 

 Helmholtz to be merely the ways along which the vibrations 

 are transmitted from the basilar membrane to the nerve end' 

 cells. 



According to Helmholtz, the long, stiff hairs of the ampullae 

 are extremely well adapted for vibrating sympathetically with 

 the motion of the fluid, and hence for producing mechanical 

 irritation in the free nerve fibres which lie in the soft epithe- 

 lium between their roots, as he supposed. 



In the utriculus and sacculus, on the other hand, the short 

 sensory hairs, according to Helmholtz, project into the concavo- 

 convex covers formed of otoliths and some undetermined matter, 

 where they are stimulated by contact with the otoliths. Helm- 

 holtz appeared to lay greater stress upon the direct action of 

 the otolithic mass upon the bare nerves ; for he says {loc. cit. p. 

 139) : "Close to these surfaces {i.e. the epithelial surfaces of the 

 maculae utriculi and sacculi), which are covered with nerves, 

 lie the calcareous concretions called auditory stones. In human 

 beings the otoliths are free (i.e. not inclosed in a solid mass). 

 These otoliths are also extremely well suited for producing 

 mechanical irritation of the nerves whenever the fluid in the 

 labyrinth is suddenly agitated. The fine light membrane with 

 its interwoven nerves probably instantly follows the motion of 

 the fluid, whereas the heavier crystals are set more slowly in 

 motion, and hence also yield up their motion more slowly, and 

 thus partly drag and partly squeeze the adjacent nerves. This 

 would satisfy the same conditions of exciting nerves, as Hei- 

 denhain's tetanomotor, which consists of a rapidly oscillating 

 ivory hammer, which at every blow squeezes without bruising 

 the nerve." 



