556 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



been in synaptic connection with path\va\s deri\cd 

 from the opposite labyrinth (65). 



iNADEquATE STIMULATION. Thc icsults obtained by 

 using various forms of inadequate stimulation have 

 supported the assumption that the flow of endolymph 

 stimulates the cristae. Thus, the old experiment of 

 Ewald with his ' pneumatic hammer' illustrates how 

 the semicircular canals are stimulated. He was able 

 to stimulate each canal separately in a pigeon by in- 

 creasing or decreasing the pre.ssure of the endolymph. 

 Two small holes were made in an osseous semicircular 

 canal near its smooth end. The hole farther from the 

 ampulla was sealed with amalgam so as to block the 

 membranous canal completely. The pneumatic ham- 

 mer, a small metal cylinder with a moving piston, was 

 cemented in the hole between the plug and ampulla. 

 Compression or decompression of the endolymph 

 caused an ampullopetal and ampullofugal endolymph 

 flow, respectively. An increase in pressure in the hori- 

 zontal canals caused the head and eyes to move 

 toward the opposite side; decompression caused a 

 weaker reaction in the reverse direction. Compression 

 and decompression of the two vertical canals cause 

 similar movements, but the effect of ampullofugal 

 flow is more marked. 



CALORIG STIMULATION. By this form of stimulation 

 movements in the endolymph are produced (7). On 

 irrigating the external auditory meatus with hot and 

 cold water, labyrinthine reactions appear becau.se 

 convection currents are provoked in the endolymph 

 of that semicircular canal which is placed in a vertical 

 position and changes in the pressure on the ampulla 

 result, cau.sing the cupula to bend. The direction of 

 the convection currents depends upon changes in the 

 specific gravity of the endolymph resulting from heat- 

 ing or cooling. Thus irrigating the car with cold 

 water causes currents toward the ampulla of a vertical 

 semicircular canal; on irrigation with warm water 

 the endolymph ri.ses. The effect of cold water is there- 

 fore the opposite of that of hot. The caloric test used 

 in clinical otology and physiological experiments has 

 the advantage over rotatory stimulation in that it 

 permits the examination of one ear at a time. If the 

 head is held in various po.sitions, any one of the three 

 semicircular canals can be stimulated; however, the 

 posterior canal, lying deep in the bone, is influenced 

 only slightly. Hot or cold water causes a greater 

 change in the temperature of the endolymph in the 



part of the canal lying nearer to the external meatus 

 than in the part more deeply situated. The tempera- 

 ture change first reaches the horizontal canal (23). 



When the head is inclined 60° backward, the 

 horizontal canals are brought into a vertical position. 

 Irrigation of the left ear of a subject with warm water 

 or the right ear with cold water produces involun- 

 tary, rhythmical conjugate deviations of the eyes 

 (nystagmus) to the right and a tendency to fall to the 

 same direction. The nystagmus appears after a short 

 latency and lasts for a varying time according, inter 

 alia, to the temperature employed and the duration 

 of the irrigation. 



A direct effect of thermal stimulation upon the 

 peripheral nerve endings, in addition to the indirect 

 effect based on movements of the endolymph, can not 

 always be excluded (38). This is in accordance with 

 the assumptions made by Bartels (8) and Breuer (11) 

 that cold water may lead to a direct paralysis of the 

 nerve endings and by Spiegel & Aronson (102) who 

 found that the nystagmus due to continued caloric 

 stimulation was independent of the position of the 

 head. 



GALV.-VNIG STi.MULATiON. Another way to elicit reflexes 

 from the labyrinth is by applying direct or alternating 

 currents to the ear. Galvanic polarization produces 

 impulse discharges similar to those occurring on nat- 

 ural rotatory stimulation (64). No movements of the 

 cupula will occur during this form of stimulation 

 (108). In the employment of this method of inade- 

 quate stimulation of the cristae or the peripheral 

 nerve fibers themselves, an electrode is placed on one 

 of the mastoids, another electrode on a distant part 

 of the body (monaural stimulation) or on the other 

 mastoid (binaural stimulation). In the latter case all 

 six canals will be stimulated owing to the current 

 spread. The galvanic stimulation will give rise to a 

 mixture of horizontal and rotatory eye movements. 

 When the cathode is on the right mastoid, the nys- 

 tagmus is to the right and vice versa. A reflex move- 

 ment of the head to the left will result if the cathode 

 of the circuit is applied to the right mastoid. 



Action of Otolith Organs 



The anatomical, physiological and physical factors 

 involved in the stimulation of thc maculae are some- 

 what different to those influencing the semicircular 

 canals. Breuer (12) realized that, although the endo- 

 lymph is not in motion when the head is at rest, we 



