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TEXT-BOOK OF PHYSIOLOGY. 



which point its tendon passes through a cartilaginous pulley, after which 

 it is reflected backward to be inserted into the superior surface of the sclera 

 about 1 6 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 oj fixation or line oj 

 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 

 axis, around which the movements of elevation (up to 34 degrees) and de- 

 pression (down to 57 degrees) 

 take place. At right angles to 

 both of these lines there is 

 the vertical axis, around 

 which the movements of ad- 

 duction (toward the nose up 

 to 45 degrees) and abduction 

 (toward the temple up to 42 

 degrees) occur. The six mus- 

 cles 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 before mentioned name- 

 ly, with the vertical axis 

 thus moving the ball only in- 

 wardly or outwardly respec- 

 tively. The other two pairs, 

 and their movements of the 



FIG. 334. MUSCLES OF THE EYE AND TENDON 

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

 ternal rectus divided. 3. Internal rectus. 4. In- 

 ferior rectus. 5. Superior rectus. 6. Superior 

 oblique. 7. Pulley for superior oblique. 8. In- 

 ferior oblique. 9. Levator palpebrae superioris. 

 10,10. Its anterior expansion, n. Optic nerve. 

 (Sappey.) 



however, have their own axes of action, 

 ball must be, therefore, analyzed with regard to all the three axes, each 

 of these four muscles producing rotation, elevation, and depression, and 

 abduction or adduction. The superior and inferior recti muscles, form- 

 ing one pair, move the eye around a horizontal axis which intersects the 

 median plane of the body in front of the eyes at an angle of 63 degrees, 

 and the superior and inferior oblique muscles forming the third pair rotate 

 the globe around a horizontal axis which cuts the median plane of the body 

 behind the eyes at an angle of 39 degrees. Thus it is that each muscle 

 moves the eye as follows, the movement for practical purposes being referred 

 to the cornea : The rectus externus draws the cornea simply to the temporal 

 side, the rectus internus simply to the nose; the superior rectus displaces the 

 cornea upward, slightly inward, and turns the upper part toward the nose 



