THE SENSE OF SIGHT. 



631 



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 (Fig. 307). 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 tendon passes through a cartilaginous pulley, after 

 which it is reflected backward to be inserted into the superior sur- 

 face 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 

 o r axes 

 have their 

 the point of rota- 

 tion 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 visual line or visual axis. 

 Around this antero-posterior axis the eye may be regarded as perform- 

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

 joining the center of rotation of both eyes, is the horizontal or 

 transverse axis, around which the movements of elevation (up to 

 34 degrees) and depression (down to 57 degrees) take place. At 

 right angles to both of these lines there is the vertical axis, around 

 which the movements of adduction (toward the nose up to 45 degrees) 

 and abduction (toward the temple up to 42 degrees) occur. The six 

 muscles may be divided into three pairs, each of which has a common 

 axis around which it tends to move the eyeball. But only the common 

 axis of the internal and external recti coincides with one of three axes 



which 

 origin at 



FIG. 307. MUSCLES OF THE EYE. TENDON OR 

 LIGAMENT OF ZINN. i. Tendon of Zinn. 2. Ex- 

 ternal rectus divided. 3. Internal rectus. 4. Inferior 

 rectus. 5. Superior rectus. 6. Superior oblique. 

 7. Pulley for superior oblique. 8. Inferior oblique. 

 9. Levator palpebrse superioris. 10, 10. Its anterior 

 expansion, n. Optic nerve. (Sappey.) 



