VESTIBULAR MECHANISMS 



553 



sacs, the two sets of end organs are, as described, 

 clearly different in detailed construction and they 

 function in accordance with somewhat different 

 principles. Interpretation of the particular functional 

 role of the different sensory endings of the labyrinth 

 has been exceedingly difficult because of the minute- 

 ness of the organ and the extreme inaccessibility of 

 the structures. The recording of action potentials 

 from the peripheral nerve or from the central nervous 

 system in response to vestibular stimulation can, in 

 many cases, serve as a revealing index of the validity 

 of the older theories presented during the last century. 

 In addition, this technique has been of considerable 

 importance in furthering the study of the mode of 

 action of the labyrinthine sensory endings (i, 6, 27, 

 38,58,68,80,94,123). 



Action oj Semnucular Canals 



The semicircular canals respond to any one of the 

 following forms of adequate and inadequate stimula- 

 tion: a) rotation (angular stimulation) of the head in 

 a vertical, transverse or anteroposterior axis; ^) arti- 

 ficial mechanical stimulation; c) caloric, irrigation of 

 the ear with hot or cold water; and </) galvanic 

 stimulation. 



ADEQUATE STIMULATION. The anatomical fact that 

 three semicircular canals are arranged in planes ap- 

 proximately at right angles to one another corre- 

 sponds with the conclusion that their function is 

 concerned with movements in the three dimensions. 

 It is generally accepted that the cristae ampullares 

 are receptors for the perception of rotatory move- 

 ments. 



The position of the cupulae is influenced by an 

 increase or decrease of velocity of rotation, i.e. by 

 positive or negative angular acceleration, but they 

 are proi^ablv not influenced by linear acceleration 

 (1,53,81,88). 



Several different theories have been presented dur- 

 ing the last century to explain the physical changes 

 in the canals resulting in stimulation of the receptor 

 cells (the hydrostatic, hydrodynamic and pressure 

 theories). Some arc today only of historical interest. 

 The literature for the first quarter of the twentieth 

 century has been fully reviewed (13, 78, 85, 122) and, 

 in addition, a number of reviews dealing with more 

 recent studies have appeared (32, 104, iio, 119). 

 According to the hydrodynamic theory of Mach, 

 Breuer and Clrum Brown, the only way in which the 

 elastic cupular ridge may be swayed, one way or 



another, is by the flow of endolymph. Any change in 

 speed of rotation will cause a deflection of the cupula 

 and the hairs of the sensory cells by a movement of 

 the endolymph with a resulting differential push and 

 pull upon the hairs. Owing to inertia, the endolymph 

 of the involved pair of canals lags behind the progress 

 of the wall of its containing tube and therefore exe- 

 cutes a movement opposite to the direction of turn- 

 ing. The speed of endolymph movement in a semi- 

 circular canal during increa.sed acceleration and the 

 resulting deviation of the cupula have been calculated 

 (95, 96, 106, 108, 109). Steinhausen (107, 108, 109) 

 was able to demonstrate that the cupula, spreading to 

 the sides and reaching to the roof of the ampulla, 

 glides during its deflection in a swing-door fashion 

 with a minimum of endolymph leakage. Some authors 

 hesitate to accept the hydrodynamic theory because 

 of the capillary nature of the canals and the viscosity 

 of the endolymph (48, 73, 78). According to Maier & 

 Lion (77), however, endolymphatic circulation is 

 possible in the minute canals. 



The hydrodynamic theory is strongly supported by 

 experiments with direct observations on the exposed 

 semicircular canals in fish. Through the injection of 

 Chinese ink into the canals of the pike, which are 

 relatively large and accessible, Steinhausen (107, 108, 

 109) was able to make visible the endolymphatic 

 current with its corresponding deflection of the cupula. 

 Dohlman (24) introduced a drop of oil into the canal 

 and the fish (cod) was rotated while the behavior of 

 the cupula was studied. As the rotation begins the 

 endolymph in the canal moves, as shown by the shift 

 in the position of the drop of oil, and the cupula 

 becomes bent over in the direction of the endolymph 

 movement (fig. 5). By using direct manometric meas- 

 urement he found cupular movement from pressure 

 changes equal to 0.05 ml of water (0.00004 gm)- 



The most effective stimulus to each ampulla is rota- 

 tion of the head in the plane of its canal. But angular 

 acceleration about any axis that lies obliquely to this 

 plane may also tend to disturb the internal liquid 

 (69, III). A more or less combined stimulation of the 

 ampullar cristae may be expected by movements of 

 the head in any one of the intermediate planes. The 

 utricle is shared by the three canals. Therefore, the 

 question arises whether this does not cause an inter- 

 ference between the canals. Indeed, when the fluid 

 in one canal is strongly affected by an acceleration, 

 part of it mav flow through into another canal. The 

 other canals arc, however, a shunt with a high re- 

 sistance, so that the leakage is small (17, 20); and, 

 for example, when angular stimulation produces 



