OTOLITH SACS OF THE INNER EAR 473 



room; from which it appears that the optical apparatus constitutes the com- 

 pensating medium. H. Munk found on monkeys that after section of all the 

 nerves to one anterior extremity, the insensitive arm could be extended for food 

 on the same day of the operation, but the hand could not be moved. During the 

 following days the number and extent of isolated movements steadily increased 

 (more rapidly so with practice), and in a few days the animal could again grasp 

 bits of food and convey them to its mouth. After some months the arm was 

 used for almost all isolated acts, but continued to be more impulsive and cum- 

 brous in its movements than the normal arm, which as time went on came to 

 be used first on most occasions. The associated movements of the arm in 

 walking, jumping, climbing, etc., however, were entirely, or almost entirely 

 obliterated; at all events they were no longer used to any advantage. 



We may sum up by saying that the messages conveyed by the afferent fibers 

 to the central organs are of great importance not only for the coordination 

 of movements but for the movements themselves, and that this depends pri- 

 marily on the fact that it is through these messages that the individual learns 

 to what extent the intended movement was carried out or failed to be carried 

 out. The nerves of the organ itself are the ones most directly concerned, 

 but they can be replaced to a greater or less extent by other nerves as, e. g., 

 the optic. When this compensation also fails the motor disturbance is greater 

 than ever and it is conceivable at least that if all afferent impulses were 

 completely inhibited, purposeful motor functions would no longer be possible. 



3. THE SEMICIRCULAR CANALS AND OTOLITH SACS 

 OF THE INNER EAR 



Physiological experiment and clinical experience both seem to have shown 

 definitely that the afferent nerves of the semicircular canals and otolith sacs 

 in the internal ear convey impulses to the nerve centers, which have much to 

 do with the perception of position or changes in the position of the head as 

 well as with other processes of orientation, etc. 



We shall investigate these phenomena without for the present raising the 

 question of how far the impulses give rise to conscious sensations. 



A. ANATOMICAL 



It is not our intention to describe the internal ear fully in this place; we 

 shall only mention briefly the structural relations which are important for 

 our present purpose. The internal ear can be divided into two portions, the 

 cochlea and the semicircular canals, together with ihe.sacculus and utriculus. 

 These two divisions have as a matter of fact entirely different functions. 



The cochlea unquestionably represents the end organ of those nerve fibers 

 the excitation of which arouses auditory sensations, particularly those of musi- 

 cal tones. This is especially well borne out by the facts of comparative anat- 

 omy. In fishes the cochlea is represented only by a very small knoblike 

 appendage to the saccule called the lagena. In frogs and toads the cochlea 

 reaches a somewhat higher development and in the reptiles a regular pro- 

 gression of stages can be followed from the turtles and snakes to the lizards 



