44^ Benj. C. Gruenherg 



of the head are such as tend to keep the plane of the mouth 

 horizontal; that is, the animal raises the head on the side that is 

 lowered, and vice versa. (At the same time there is a contraction 

 in the limbs on the ascending side and a corresponding flexion on 

 the descending side.) There can, of course, be no question of a 

 nystagmus in these two cases, since the rotation cannot be con- 

 tmued beyond a small arc of a circle without dislodging the animal 

 or causing it to make definite efforts to hold its own, and in neither 

 case are there compensatory responses to rotation. Nor are the 

 responses normal if the animal is fastened to the support. 



When a frog is moved about in a circle having a diameter of 

 two to three meters (by walking about with a jar containing the 

 animal), the animal turns its head away from the center if the frog 

 faces the direction of motion, and toward the center if the animal 

 is carried facing in the direction opposite to that of the movement, 

 that is, backward. This movement and the response are virtually 

 the same as in the case of rotation about a vertical axis, in a large 

 circle and at a slow rate. 



The movement of the head in the cases referred to is in general 

 in a direction opposite to that of the displacement of the body. 

 Such responses have long been known in the frog^ as well as in 

 other animals, and are frequently spoken of as "compensatory 

 movements. " The implication of this designation, as well as 

 the expressed belief of many physiologists, is that the movements 

 in question are in some way related to the orientation of the ani- 

 mal with regard to gravity, or, what is mechanically equivalent, 

 to acceleration of motion in some direction. The movements have 

 been regarded as reflexes set up by the sensation of the semicir- 

 cular canals. 



2 THEORIES OF THE FUNCTION OF THE SEMICIRCULAR CANALS 



The oldest theory as to the function of the semicircular canals 

 was that they were concerned in the perception of the direction 

 of sound, and was deduced from their intimate anatomical asso- 



' Some of these responses seem first to have been described by Goltz ('69, p. 71), and many of them 

 have been shown by Steiner ('85, p. 126) to take place in frogs whose fore- and mid-brain have been 

 removed. 



