450 PHYSIOLOGY 



of the animal is well shown if the otolith be replaced by a small 

 fragment of iron. Under normal circumstances the iron particle will 

 act quite as well as an otolith. If, however, a powerful magnet be 

 brought in the neighbourhood of the animal, the pressure of the 

 particle will not be determined simply by gravity and therefore by 

 the position of the animal, so that there will be a dissonance between 

 the impulses arriving from the otolith organ and those arising from 

 the sense-organs of the body, and marked disorders of equilibrium 

 are the result. 



In the saccule and utricle the vestibular nerve ends in similar 



a 



otolith organs known as the maculas acousticae. Each of these is a 

 small elevation covered with long hairs and supplied with nerves. 

 One or two calcareous secretions or otoliths are embedded in the 

 hairs, so that any change in position will cause a corresponding change 

 in the nerve fibres which are being excited by the weight of the 

 otoliths. The semicircular canals, which lie in the three planes of 

 space, are also provided with end-organs, somewhat similar in struc- 

 ture to the maculae acousticae, but devoid of otoliths. They are 

 excited by mass movements of the fluid endolymph, filling the 

 canals, which are set up by rotation of the head. 



Since the nervous apparatus of the labyrinth is excited not by 

 changes in the environment, from which it is carefully shielded, but 

 by changes in the animal itself, we are justified in assigning it to 

 the proprioceptive system, of which indeed it represents the most 

 important receptor. Just as the proprioceptive nerves of a limb 

 are responsible for the tonus of the limb muscles, so, as Ewald has 

 shown, each labyrinth is responsible to a considerable degree for 

 the tonus of the corresponding side of the body. Extirpation of 

 one labyrinth causes a lasting loss of tone in the muscles of the same 

 side. A further functional resemblance lies in the part played by 

 the labyrinth in the determination of posture. The resultant effect 

 of the impulses arisir in it is to maintain a reflex posture of the 

 head and eyes, so that the optic axes in a position of rest are directed 

 towards the horizon. Stimulation of the labyrinth causes therefore 

 movements of the eyes which may or may not be associated with 

 correlated movements of the head. 



As in the case of the other sense-organs of the anterior end of 

 the body, the reflexes excited from the labyrinth dominate over 

 those evoked by proprioceptive impulses from the hinder portions 

 of the body. At the entry of its nerve into the brain stem a mass 

 of grey matter is developed, which must be regarded as the head 

 ganglion of the proprioceptive system, and the chief co-ordinating 

 organ of all the reflex systems which determine posture of the limbs 

 and of the whole animal, and therefore the maintenance of equilibrium 



