270 OUTPOSTS OF THE INTELLIGENCE SERVICE 



than ordinary ciliated epithelium {q.v.), and held together by a 

 nnicous gelatinous mass so that they cannot move freely in the 

 endolymph. The hair-cells are supplied with fine filaments from 

 the vestibular division of the eighth nerve. 



The ducts on each side open into the corresponding utricle by 

 five orifices, i.e. two of the ducts (the anterior and the posterior) 

 join in a common canal {cms commune) and have a common opening 

 into the utricle. 



Mechanism. The structure of the organ indicates that altera- 

 tions of the pressure of the viscous fluid in the membranous 

 ampullae will tend to bend the hairs and, following the same 

 scheme as in the maculae, will alter muscle tone. When a rotatory 

 movement round any axis is initiated or accelerated positively or 

 negatively, positive or negative fluid pressure beyond the normal 

 will tend to develop in certain ampullae and so produce stimulation 

 of the cristae. Initiation of rotation, for instance, of an animal 

 about its dorso-ventral axis, i.e. in the plane of the horizontal 

 canals, produces t'ne same results as mechanical stimulation of the 

 hairs with a tiny pledget of cotton wool. Adaptation occiu-s 

 gradually on continuing the rotation at a steady rate. That is, 

 like the proprioceptors already studied, adaptation is a slow 

 process. Any alteration of rate, quickening, slowing or stopping 

 acts as a fresh stimulus. 



It has been shown conclusively by Mach that actual currents 

 cannot be produced either in the perilymph of the canals or in 

 the endolymph of the ducts. Maxwell has disposed of the theory 

 that fluid pressure is developed in the duct and transmitted to the 

 ampulla. He tied off the posterior end of the external canal 

 (horizontal) and, after cutting it, raised it so that its plane was 

 vertical. He then rotated the animal about its dorso-ventral axis 

 and produced the usual reactions. That is, the animal reacted 

 normally when it was possible only for fluid to enter the ampulla 

 from the utricle. The normal mechanism, therefore, for excitation 

 of the nerve-endings in the cristae is the transmission of fluid-pressure 

 from the utricle to the ampullae. This pressure is prevented from 

 rapid dissipation by the narrowing of the duct by eight times as it 

 leaves the ampulla. 



The otoliths of the maculae and the hairs of the cristae are both 

 capable of responding to angular and linear acceleration. Maxwell 

 has shown that the otoliths, because of their greater specific 

 gravity, have a greater inertia than the fluid they displace and, 

 therefore, are as likely to suffer displacement during rotation as the 

 endolymph. There is also a physical possibility that during linear 

 and angular accelerations the utricular fluid would tend to lag and 



