940 



THE SENSORY APPAEATUSE8. 



3. Great Oblique Muscle (trochUaris, or vbUquus superior ocuh').—Ljina 

 to the side of the internal and superior rectus, and formed, like them, of a fleshy 

 band terminated by a thin aponeurosis, this muscle differs from the preceding in 

 its interrupted couree. Arising from the back of the orbit, and passing forward 

 ao-ainst the inner wall of that cavity, it reaches a strong tibro-cartilaginous, 

 pully-like process — a dependency of the aponeurosis of the orbit— attached by 

 its extremities to the frontal bone, at the base of the orbital process ; it passes 

 through this loop, and then bends outwards, to insinuate itself below the terminal 

 extremity of the superior rectus, and become inserted into the sclerotic, between 

 the latter muscle and the external rectus. 



This muscle pivots the eye inwards and upwards in the orbit, carrying the 



outer aspect of the globe upwards. 



Fig. 510. 



MUSCLES OF THE EYEBALL (VIEWED FROM ABOVE). 



1, Section of orbital process of frontal bone to which 

 the fibro-cartilaginous pulley, 4, of the superior 

 oblique muscle, 5, is attached ; 2, zygomatic 

 process of the temporal bone ; 3, (lortion of sphe- 

 noid bone into which the recti and superior oblique 

 muscles are implanted; 6, pathetici nerve; 7, 

 internal rectus ; 8, superior rectus ; 9, levator 

 palpebrfe muscle; 10, external rectus; 11, eye- 

 ball; 12, upper eyelid; 13, lower eyelid; 14, inner 

 canthus of eye. 



and its lower part outwards ; this 

 faculty it owes to its reflection in 

 the cartilaginous loop, as it acts as 

 if its insertion were at the angle it 

 forms there. 



4. Small Oblique Muscle 

 {obliquus inferior ocidi). — Much 

 thicker, though very much shorter 

 than the preceding, and almost 

 entirely fleshy, this muscle is placed 

 in a transverse direction on the 

 globe of the eye, being nearly 

 parallel to the reflected portion of 

 the great oblique. It arises in the 

 lachrymal fossa, passes outwards, 

 and terminates in the sclerotic, be- 

 tween the external and inferior recti 

 muscles. 



It is an antagonist of the great 

 oblique, pivoting the eye in a con- 

 trary direction. 



It is to be noted that the double 



rotatory movement executed by the 

 oblique muscles is altogether involuntary, and that it is constantly produced when 

 the animal inclines its head to one side — doubtless to maintain the visual axis 

 always in identical relations with the same point of the retina. This movement 

 is well seen in Man when the head is brought round to either shoulder : the eye 

 then pivots in the orbit in an inverse direction to that to which the head inclines, 

 so that a mark placed at the upper part of the iris when the head is straight 

 would occupy the same position after the lateral movement. Simultaneous in 

 both eyes, this pivoting is executed by certain muscles in each ; the great oblique 

 for one, the small oblique for the other, according to the direction in which the 

 head is turned. 



(A third, or middle oblique muscle, has been mentioned by Strangeways, as 

 sometimes, if not always, found between the superior and inferior oblique muscles. 

 It has been described as arising by a fine tendon from a small depres^^ion in 

 the upper part of the orbital process of the frontal bone, between the origin of 

 the inferior oblique and the pulley of the superior oblique muscle. This tendon 



