MUSCLES OF THE OEBIT. 275 



MUSCLES OF THE OBBIT. 



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 longitudinally forwards 

 to their insertion in front. 



The levator palpeTbrra superioris is a slender muscle, which arises, 

 pointed and tendinous, above and in front of the margin of the optic 

 foramen, and, passing forwards over the eyeball, ends in a fibrous 

 expansion inserted into the anterior surface of the tarsal cartilage of 

 the upper eyelid. 



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

 fourth and frontal nerves and the supraorbital vessels, and beneath it are the 

 superior rectus muscle and the globe of the eye. In the lid, it is placed behind 

 the pal^jebral ligament, close to the membrana conjunctiva. 



The four straight muscles of tlie eye surround at their origin the optic 

 nerve, and, passing forwards from that point, are inserted into the 

 front of the globe of the eye at four opposite equidistant parts, by 

 delicate expanded tendons which become blended into one at their 

 termination. 



The superior rectus arises close in front of the foramen opticum, 

 and beneath the levator palpebrjB; the inferior rectus, internal 

 rectus, and external rectus are united in a common tendinous attach- 

 ment around the circumference of the optic foramen, except above. 

 But the external rectus differs from the others in having two heads of 

 origin : the upper head unites with that of the superior rectus, the 

 second head arises from the common tendon and from a bony point on 

 the lower margin of the sphenoidal fissure close to its wider inner 

 end ; and intermediate fibres proceed from a fibrous band between the 

 heads of origin. The four recti thus attached posteriorly, pass for- 

 wards, one above, one below, and one on each side of the eyeball, 

 becoming flattened as they lie in contact with it, and are inserted by 

 short membranous tendons into the fore part of the sclerotic coat, at 

 an average distance of four lines from the margin of the cornea. 



In length and breadth there are some differences among these muscles. The 

 external rectus exceeds the internal one in length. On the other hand, the 

 internal rectus is the broadest, and the superior the naiTowest of all. Between 

 the heads of the external rectus is a naiTOw interval, which gives transmission to 

 the third and sixth nerves and the nasal branch of the fifth nerve, with the 

 ophthalmic vein. 



The superior oblique or trochlearis is a narrow elongated muscle, 

 placed at the upper and inner part of the orbit, internally to the levator 

 palpebral. It arises about a line in front of the inner part of the optic 

 foramen ; thence it proceeds towards the internal angle of the orbit, 

 and terminates in a round tendon which passes through a fibro-carti- 

 laginous ring or pulley (trochlea) attached to the fovea trochlearis of 

 the frontal bone ; it is there reflected outwards and backwards, and 

 passes between the eye and the superior rectus to be inserted into the 

 sclerotic coat midway between the superior and external recti muscles, 

 nearly equidistant from the cornea and the entrance of the optia 



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