CH. LV.] MOVEMENTS OF THE EYEBALL. 781 



upon the retina is the movement of the pigment cells. On being 

 stimulated by light the granules of pigment in the cells which 

 overlie the outer part of the rod and cone layer of the retina 

 pass down into the processes of the cells, which hang down 

 between the rods : these melanin or fuscin granules are generally 

 rod-shaped, and look almost like crystals. In addition to' this a 

 movement of the cones and possibly of the rods occurs, as has 

 been already incidentally mentioned ; in the light the cones 

 shorten and in the dark they lengthen (Engelmann). 



According to the careful researches of Dewar and McKendrick, 

 and of Holmgren, it appears that the stimulus of light is able 

 to produce a variation of the natural electrical currents of the 

 retina. The current is at first increased and then diminished ; 

 this is the electrical expression of those chemical changes in the 

 retina of which we have already spoken. 



The electrical variations in the retina under the influence of light have 

 been recently reinvestigated by Waller. The excised eyeball of a frog is 

 led off by non-polarisable electrodes to a galvanometer. One electrode is 

 placed on the front, the other on the back of the eye. A current of rest 

 (ilrmarcatinn current) is observed passing through the eyeball from front to 

 back. When light falls on the eye this current is increased ; on shutting off 

 the light there is a momentary further increase, and then the current slowly 

 returns back to its previous condition. Waller explains this by supposing 

 that anabolic changes in the eye predominate during stimulation by light. 

 With the onset of darkness, the katabolic changes cease at once, and the 

 anabolic more slowly ; hence a further positive variation. If the eyeball 

 has been excised the day before the observations are made, or has been 

 fatigued or injured, light produces principally katabolic changes, as evidenced 

 by a negative variation. A slight positive effect follows when the light is 

 shut off. 



MOVEMENTS OF TriE EYEBALL. 



Protrusion of the eyeballs occurs (i) when the blood-vessels of 

 the orbit are congested ; (2) when contraction of the plain 

 muscular fibres of the capsule of Tenon takes place ; these are 

 innervated by the cervical sympathetic nerve; and (3) in the 

 disease called exophthalmic goitre. 



Retraction occurs (i) when the lids are closed forcibly; (2) 

 when the blood-vessels of the orbit are comparatively empty ; 

 (3) when the fat in the orbit is reduced in quantity, as during 

 starvation ; and (4) on section or paralysis of the cervical 

 sympathetic nerves. 



The most important movements, however, are those produced 

 by the six ocular muscles. 



The eyeball possesses the power of movement around three axes indicated 

 in fig. 593. viz. an antero-posterior, a vertical, and a transverse, passing 

 through a centre of rotation a little behind the centre of the optic axis. 

 The movements are accomplished by pairs of muscles. 



