480 ORGANIC SENSATIONS 



and fro and the eyes also take part in the movements. Now it is a probability 

 supported by many facts that the first phase of the movement represents a 

 reaction of the animal to the rotation; the second phase is produced, because 

 after the head has been carried far enough from the median position the 

 afferent impulses from the joints, muscles., etc., are strong enough to arouse 

 the opposite sets of muscles. 



When the two external canals are plugged up so as to prevent movements of 

 the fluid, the reaction to rotation in the horizontal plane is almost entirely want- 

 ing. On the other hand, one can destroy any number of the anterior and pos- 

 terior semicircular canals without changing the reaction. 



The characteristic eye movements occur also when mammals are rotated; 

 they are wanting after section of the eighth cranial nerve or of the semicircular 

 canals. 



From these facts we may conclude that the semicircular canals are influ- 

 enced by movements of the head, and in all probability the immediate stimulus 

 is caused by currents set up in the endolymph; this means that the sensory 

 hairs of the corresponding crista acustica are put on the stretch and the ap- 

 propriate end organs are consequently excited. These in their turn produce 

 reflex responses by which the position of the head and of the eyes is regulated. 



The movements of the eyes and of the head which have been seen to take 

 place when the animal is rotated may appear after extirpation of the labyrinths 

 when the eyes of the animal are open. At the beginning of rotation the animal 

 experiences a displacement of the retinal picture, and seeks to resist that dis- 

 placement by striving to hold the object steadily in view. 



The subsequent motion of the head in the direction of the rotation is pure 

 reflex, probably discharged by the excitation of the retina due to the displace- 

 ment of the image or by impulse? coming from the neck and eye muscles. 



The effects of extirpation of the semicircular canals which have been sum- 

 marized under B can be brought into line with these results without serious 

 difficulty. Suppose a normal animal moves his head, say, to the right. The 

 movement produces in both external semicircular canals currents in the endo- 

 lymph in the opposite direction, and these in turn reflexly induce a rotation of 

 the head in the direction of the current i. e., in the direction reverse to the 

 original rotation of the head. If the two external canals are now thrown out 

 of function, either by being plugged or by being sectioned, the currents are not 

 set up and consequently the muscular movements caused by them do not take 

 place. But the head may swing to one side. If so, it will continue until the 

 motor sensations from the joints, etc., discharge compensating movements. This 

 mode of discharge, however, is not so finely graded, for the animal has lost the 

 power of telling when the head has been returned to normal position; conse- 

 quently it is difficult for the animal to regain its equilibrium after that has 

 once been disturbed. When several canals are suppressed at the same time the 

 swinging motions of the head become still more extensive. 



The laxness of the muscles which has been observed affects most the muscles 

 of greatest precision e. g., the extrinsic muscles of the eyes and is probably 

 due to the absence of impulses normally roused by the labyrinth (Jensen). 



