746 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



of vision. 1 But all movements of the eye from the primary position take place, 

 as we have seen, round an axis lying in this plane. Hence all such movements 

 must be produced by more than one muscle, and this circumstance also is prob- 

 ably of advantage in estimating the extent and direction of the movement. In 

 this connection it is interesting to note that the eye-muscles have an exception- 

 ally abundant nerve-supply a fact which it is natural to associate with their 

 power of extremely delicate adjustment. It has been found by actual count 

 that in the muscles of the human eye each nerve-fibre supplies only two or three 

 muscle-fibres, while in the muscles of the limbs the ratio is as high as 1 to 

 40-1 25. 2 



Although each eye has its own supply of muscles and nerves, yet the two 

 eyes are not independent of each other in their movements. The nature of 

 their connections with the nerve-centres is such that only those movements are, 

 as a rule, possible in which both axes of vision remain in the same plane. This 

 condition being fulfilled, the eyes may be together directed to any desired point 

 above, below, or at either side of the observer. The axes may also be con- 

 verged, as is indeed necessary in looking at near objects, and to facilitate this 

 convergence the internal recti muscles are inserted nearer to the cornea than the 

 other muscles of the eye. Though in the ordinary use of the eyes there is never 

 any occasion to diverge the axes of vision, yet most persons are able to effect a 

 divergence of about four degrees, as shown by their power to overcome the ten- 

 dency to double vision produced by holding a prism in front of one of the eyes. 

 The nervous mechanism through which this remarkable co-ordination of the 

 muscles of the two eyes is effected, and their motions limited to those which 

 are useful in binocular vision, is not completely understood, but it is supposed 

 to have its seat in part in the tubercula quadrigemina, in connection with the 

 nuclei of origin of the third, fourth, and sixth cranial nerves. Its disturbance 

 by disease, alcoholic intoxication, etc. causes strabismus, confusion, dizziness, 

 and double vision. 



A nerve termination sensitive to light, and so arranged that it can be turned 

 in different directions, is sufficient to give information of the direction from 

 which the light comes, for the contraction of the various eye-muscles indicates, 

 through the nerves of muscular sense, the position into which the eye is nor- 

 mally brought in order to best receive the luminous rays, or, in other words, 

 the direction of the luminous body. The eye, however, informs us not only of 

 the direction, but of the form of the object from which the light proceeds ; and 

 to understand how this is effected it will be necessary to consider the refracting 

 media of the eye by means of which an optical image of the luminous object 

 is thrown upon the expanded termination of the optic nerve viz. the retina. 



Dioptric Apparatus of the Eye. For the better comprehension of this 

 portion of the subject a few definitions in elementary optics may be given. A 



1 The axes of rotation of the internal and external recti, however, deviate but slightly from 

 the equatorial plane. 



2 P. Tergast : " Ueber das Verhaltniss von Nerven und Muskeln," Archiv fur rnikr. Anat.. 

 ix. 36-46. 



