920 CENTRAL NERVOUS SYSTEM 



these reactions. In the dogfish the positions of the eyes is also con- 

 trolled by the proprioceptors of the tail muscles, for if the tail is bent 

 from side to side compensating movements of the eye are produced of 

 the same nature as those which accompany the movements of the tail 

 in swimming (Lyon 29 ). Eye movements of a compensatory character are 

 also seen in man during rotation of the body. As the body turns the 

 eyes swing slowly in the opposite direction so as to maintain their fixa- 

 tion. Having turned as far as possible, they swing quickly back in the 

 opposite direction to fix a new object which in turn they follow by a slow 

 deviation. This slow deviation alternating with a rapid movement in 

 the opposite direction is called nystagmus. It also occurs during the sen- 

 sation of turning which persists for some seconds after the rotation of the 

 body has come to an end, and in certain pathological conditions. 



Clinical Tests of the Labyrinthine Mechanism. The influence of the 

 labyrinth on the postural coordination of the eye and limb muscles, forms 

 the basis for certain useful clinical tests by which the condition of the 

 labyrinth and of its central connections can be determined. These have 

 been developed chiefly by Barany. In normal individuals when the semi- 

 circular canals are stimulated in addition to the turning sensation and 

 nystagmus, a phenomenon known as past pointing occurs. If the sub- 

 ject is directed to close his eyes, extend his arm, raise and then lower 

 it in a vertical plane he will have no difficulty in bringing it back to its 

 original point. If he attempts to do this while the labyrinth is being 

 stimulated he will, if normal, return the arm to a position which de- 

 viates from the original point in the direction from which he thinks he 

 is turning, i. e., in the rotation test in the direction toward which he has 

 been spun. 



The methods used to excite the canals are called the rotation test and 

 the caloric test. The rotation test is performed by spinning the subject 

 about two or three times in a pivoted chair, with the head held so that 

 the pair of canals to be tested lie in the plane of rotation. The caloric 

 test depends on the fact that if warm (112 F.) or cold (68 F.) water 

 is poured into the external auditory canal it will set up convection cur- 

 rents in that semicircular canal which lies in a vertical position, and 

 there give rise to excitation. By holding the head in various positions 

 any one of the three canals can be stimulated. The caloric test has 

 the obvious advantage that it excites the canals of one labyrinth only. 

 If labyrinthine stimulation by these methods does not produce after 

 turning sensations, nystagmus, or past-pointing it is concluded that a 

 lesion occurs in the sense organ or some part of the nervous system in- 

 volved in the reaction. The past-pointing test is particularly useful, 

 since it can be applied to any joint and in any plane, and consequently 

 it has been possible to study the localization of the centers in the cere- 



