THE MUSCLES OF THE ORBIT. 289 



lower eyelid, as in mirth ; and those which ascend to the angle of the mouth depress that 

 part, as in grief. (On the action of the facial muscles may be consulted C. Bell, " Anatomy 

 and Philosophy of Expression ; " Duchenne, " Mecanisme de la Physionomie humaine," Paris, 

 1862 ; and C. Darwin. " Expression of the Emotions in Man and Animals.") 



THE MUSCLES OF THE ORBIT. 



In this group will be described seven muscles, namely, the elevator of the upper 

 eyelid before referred to, and six muscles of the eyeball, viz., the four_straight and 

 the two oblique muscles. Of these muscles, the inferior oblique alone is confined to 

 the fore part of the orbit ; all the others take their origin -at its back part, and pass 

 forwards to their insertion. 



The levator palpebrae superioris is a slender muscle which arises, pointed 

 and tendinous, above and in front of the optic foramen, from which it is separated 

 by the origin of the superior rectus. Becoming broader as it passes forwards over 

 the eyeball, it ends a little behind the supraorbital margin in a membranous expan- 

 sion which is inserted chiefly into the fibrous tarsus of the upper eyelid. 



From the membranous expansion of the levator a thin superficial layer is continued in 

 front of the tarsus to be inserted into the skin of the lid, its fibrous bundles piercing the 

 palpebral fascia and passing between the fasciculi of the palpebral portion of the orbicularis. 1 

 The layer inserted into the tarsus consists mainly of unstriped muscular fibres, and is attached 

 along the upper edge of the fibrous plate. Some deeper tendinous fibres, joining an expansion 

 from the sheath of the superior rectus, are attached to the conjunctiva. Lastly, the marginal 

 part of the aponeurosis is prolonged on each side beyond the limit of the lid, and is attached 

 to the wall of the orbit in conjunction with offsets from the sheaths of the recti ; and 

 internally a slip is fixed to the hinder part of the trochlea. 



Relations. Between this muscle and the roof of the orbit are situated the fourth and 

 frontal nerves and the supraorbital vessels ; beneath it are the inner half of the superior 

 rectus and the globe of the eye. On entering the lid, it is placed behind the palpebral fascia, 

 and its deep surface rests on the conjunctiva. 



The four STRAIGHT MUSCLES OF THE EYE have a continuous tendinous origin at 

 the bottom of the orbit, in the form of an oval ring which, commencing above, 

 passes down on the inner side to the lower margin of the optic foramen, then 

 stretches transversely across the inner part of the sphenoidal fissure to be attached 

 to its lower border, where a prominent bony point is generally developed, and finally 

 is completed by again crossing the sphenoidal fissure, this time about the middle 

 and vertically to gain the upper edge of the optic foramen. In the upper and 

 lower parts of this ring two stronger bands, or common tendons, are developed, which 

 extend forwards on the ocular surface of the muscles, and from which most of the 

 fleshy fibres spring. The upper is fixed to the inferior root of the small wing of 

 the sphenoid, between the optic foramen and the sphenoidal fissure, and is prolonged 

 anteriorly into the internal, superior, and external recti (Lockwood). The lower 

 tendon, larger and more distinct than the upper, arises from the body of the 

 sphenoid at the inner end of the sphenoidal fissure, and divides into three slips for 

 the internal, inferior, and external recti (Zinn). The superior rectus arises 

 above or in front of the optic foramen, and beneath the levator palpebrae ; the 

 internal rectus has a wide origin on the inner side of and below the optic 

 foramen ; the inferior rectus springs mainly from the middle slip of the lower 

 common tendon ; and the external rectus differs from the others in having two 

 heads of origin, the inferior and larger of which springs from the outer slip of the 

 lower common tendon and the spine on the lower border of the sphenoidal fissure, 

 while the superior is formed by the outer part of the upper common tendon ; be- 



1 F. Merkel, " Handbucli der topographischen Anatomic," i, 199. 



