132 SPECIAL SENSES. 



developed are interesting and important in their pathological 

 bearing, but the intricate mathematical calculations involved 

 are not essential to our comprehension of the simple acts by 

 which the axis of vision is moved in various directions. It is 

 not necessary to discuss the exact location of the centre of ro- 

 tation of the eye, the mathematical formulae illustrating what 

 are known as the laws of rotation, or the law of torsions, es- 

 pecially as some of these points are still unsettled, notwith- 

 standing the elaborate and delicate observations of Bonders, 

 Helmholtz, and many others. In the admirable work of 

 Helmholtz, to which we have so often referred, we find the 

 following statement, which will form the basis of our study 

 of the ocular movements : 



" By the diversified combination of the action of the six 

 muscles, not only may the axis of the eye be turned in all di- 

 rections, but the eye may, in addition, receive movements of 

 rotation around this axis. If we have assumed for each pair 

 of muscles a common axis of rotation, it is that this supposi- 

 tion seems admissible, at least as a first approximation, and 

 that it simplifies to a remarkable degree the examination of 

 the movements which the muscles of the eye have to per- 

 form." 1 



Under ordinary conditions, in the human subject, the ac- 

 tion of the six ocular muscles is confined to the movements 

 of rotation and torsion of the globe. It is said that, in the 

 human subject, there is no such thing as protrusion of the eye, 

 from general relaxation of these muscles, and that it is impos- 

 sible, by a combined action of the four recti muscles, to re- 

 tract the globe in the orbit ; 2 but those who have operated 

 upon the eyes assert positively that this statement is erro- 

 neous ; and that the globe is almost always suddenly and 

 powerfully drawn within the orbit when a painful impression 

 is made upon the cornea. This is stated as a matter of com- 

 mon observation by ophthalmic surgeons* 3 



1 HELMHOLTZ, Optique physiologique, Paris, 1867, p. 41. 



2 HELMHOLTZ, op. cit., p. 596. 3 Oral communication from Prof. Noyes. 



