THE SENSE OF SIGHT 



723 



dation and the formation of diffusion-circles, they are indistinct. For these 

 reasons they are readily neglected. 



In the primary position of the eyes that is, a position in which the 

 visual axes are parallel the horopter is a plane at infinity. In the tertiary 

 positions the horopter is a curve of complex form. 



Movements of the Eyeball. The almost spheric eyeball lies in the 

 correspondingly shaped cavity of the orbit, like a ball placed in a socket, 

 and is capable of being rotated to a considerable extent by the six muscles 

 which are attached to it. These muscles are the superior and inferior recti, 

 the external and internal recti, and the superior and inferior obliqui. The 

 four recti muscles arise from the apex of the orbit cavity, from which point 

 they pass forward to be inserted into the sclera about 7 to 8 mm. from the 

 corneal border. The superior oblique muscle having a similar origin passes 

 forward to the upper and inner angle of the orbit cavity, at which point its 



e 



FIG. 312. HOROPTER FOR THE 

 SECONDARY POSITION, WITH CON- 

 VERGENCE OF THE VISUAL AXES. 

 (Landois.} 



FIG. 313. SCHEME OF IDENTICAL AND 

 NON-IDENTICAL POINTS OF THE RETINA. 

 (Landois.} 



tendon passes through a cartilaginous pulley, after which it is reflected back- 

 ward to be inserted into the superior surface of the sclera about 16 mm. 

 behind the corneal border. The inferior oblique muscle arises from the inner 

 and inferior angle of the orbit cavity. It then passes outward, upward, and 

 backward, to be inserted into the upper, posterior, and temporal portion of 

 the sclera about 4 or 5 mm. from the optic nerve entrance. 



The movements of each eye are referred to three fixed lines or axes, 

 which have their origin at the point of rotation of the eyeball, this point 

 lying about 1.7 mm. behind the center of the globe. If the eye looks straight 

 forward in the horizontal plane (the head being erect), the line joining the 

 center of rotation with the object looked at is the line of fixation or line of 

 regard. Around this antero-posterior axis the eye may be regarded as per- 

 forming its circular rotation or torsion. At right angles to this line, and 

 joining the centers of rotation of both eyes, is the horizontal or transverse 



