MO VEMENTS OF THE E YES. 1131 



Nagel l and Hering 2 have found that certain small deviations 

 from Listing's law occur, when it is necessary, for the sake of single 

 vision, to keep the images of an external object on corresponding points. 



Compensatory eye movements. The movement of this kind 

 about which there has been most doubt, is the swivel rotation which 

 takes place when the head is inclined to one side. Such a move- 

 ment in the opposite direction to the inclination of the head was first 

 described by John Hunter. Its existence has been denied, but the 

 results of many investigations show that there is a partial compensation 

 only, the angle of rotation of the eyeball being given by different 

 observers from J to J T of the angle of inclination of the head. The 

 subject has recently been investigated by W. A. Nagel, 3 using the blind- 

 spot method. He found that the amount of compensation varied 

 from ~ to ~, the rotation of the eyeball decreasing relatively as the 

 inclination of the head increased. 



The compensatory movements of the eyeballs are influenced by the 

 condition of the labyrinth, and especially of the semicircular canals, and 

 have been much used as a means of investigating the functions of these 

 organs, both in man and other animals. The movements are abolished 

 by destruction of both labyrinths, and either appear not to be affected or 

 are exaggerated with destruction of one side. 



Nagel 4 has investigated the compensatory swivel rotation in a 

 number of animals, and with more extensive rotation of the head than has 

 been studied in man, and has found considerable differences in different 

 animals. The rabbit showed as much as 90 of rotation of the eyeball 

 round its axis ; while in birds, with great mobility of the neck, com- 

 pensatory eye movements may be completely absent. 



Associated movements. Some movements of the eyeballs are 

 more closely associated than others. The parallel movements of the two 

 eyes together are very closely associated, and not only continue when one 

 or both eyes have become blind, but were observed by Donders 5 in a 

 case of complete congenital blindness. The exact association between 

 the movements of accommodation in the two eyes has already been con- 

 sidered (p. 1038). There is a very close association between convergence, 

 accommodation, and contraction of the pupil on the one hand, and diver- 

 gence, relaxation of accommodation, and dilatation of the pupil on the 

 other ; but these movements are capable of dissociation to a considerable 

 extent. Distinct vision with convex or concave glasses depends on a 

 certain degree of this dissociation, accommodation altering while the con- 

 vergence remains constant, while, by means of a prism, convergence or 

 divergence may be made to occur without accompanying accommodation. 

 The same dissociation occurs when well-defined double or combined 

 images of objects are seen by diverging or converging the eyes. The ease 

 of dissociation may be greatly increased by practice. The amount of 

 accommodation which can take place with a given convergence of the 

 visual lines is known as the relative range of accommodation. Donders 6 

 found that with increasing convergence the relative range at first increased 

 and then decreased, till with maximum convergence it became nil. 



1 "Das Sehen mit zwei Augen," Leipzig, 1861. 2 Hermann's "Handbuch," Bd. iii. S. 504. 



3 Ztschr.f. Psychol. u. Physiol. d. Sinnesorg., Hamburg u. Leipzig, 1896, Bd. xii. S. 331. 

 See this paper for a full account of the subject. 



4 Loc. tit., S. 346. 5 Arch.f. d. ges. Physiol., Bonn, 1876, Bd. xiii. S. 283. 

 6 " Anomalies of Accommodation and Refraction," p. 113. 



