THE ORBITAL REGION. 305 



behind the equator of the eyeball, the insertion of the muscle lying between the 

 Superior and External recti. 



Relations. By its upper surface, with the periosteum covering the roof of the 

 orbit and the fourth nerve : the tendon, where it lies on the globe of the eye, is 

 covered by the Superior rectus ; by its under surface, with the nasal nerve, 

 ethmoidal arteries, and the upper border of the internal rectus. 



The Inferior oblique is a thin, narrow muscle placed near the anterior margin 

 of the orbit. It arises from a depression on the orbital plate of the superior 

 maxillary bone, external to the lachrymal groove. Passing outward, backward, 

 and upward between the Inferior rectus and the floor of the orbit, and then 

 between the eyeball and the External rectus, it is inserted into the outer part of 

 the sclerotic coat between the Superior and External recti, near to, but somewhat 

 behind, the tendon of insertion of the Superior oblique. 



Relations. By its ocular surface, with the globe of the eye and with the Inferior 

 rectus ; by its orbital surface, with the periosteum covering the floor of the orbit, 

 and with the External rectus. Its borders look forward and backward; the 

 posterior one receives a branch of the third nerve. 



Nerves. The Levator palpebrse, Inferior oblique, and all the Recti excepting 

 the External, are supplied by the third nerve ; the Superior oblique, by the fourth ; 

 the External rectus, by the sixth. 



Actions. The Levator palpebrse raises the upper eyelid, and is the direct 

 antagonist of the Orbicularis palpebrarum. The four Recti muscles are attached 

 in such a manner to the globe of the eye that, acting singly, they Avill turn it 

 either upward, downward, inward, or outward, as expressed by their names. 

 The movement produced by the Superior or Inferior rectus is not quite a simple 

 one, for, inasmuch as they pass obliquely outward and forward to the eyeball, the 

 elevation or depression of the cornea must be accompanied by a certain deviation 

 inward, with a slight amount of rotation, which, however, is corrected by the 

 Oblique muscles, the Inferior oblique correcting the deviation inward of the 

 Superior rectus, and the Superior oblique that of the Inferior rectus. The con- 

 traction of the External and Internal recti, on the other hand, produces a purely 

 horizontal movement. If any two contiguous recti of one eye act together, they 

 carry the globe of the eye in the diagonal of these directions viz. upward and 

 inward, upward and outward, downward and inward, or downward and outward. 

 The movement of circumduction, as in looking round a room, is performed by 

 the alternate action of the four Recti. The Oblique muscles rotate the eyeball 

 on its antero-posterior axis, this kind of movement being required, for the correct 

 viewing of an object when the head is moved laterally, as from shoulder to 

 shoulder, in order that the picture may fall in all respects on the same part of the 

 retina of each eye. 1 



Fasciae of the Orbit. The connective tissue of the orbit is in various places con- 

 densed into thin membranous layers, which may be conveniently described as (1) the 

 orbital fascia; (2) the sheath of the muscles; and (8) the covering of the eyeball. 



(1) The orbital fascia. This forms the periosteum of the orbit. It is loosely 

 connected to the bones, from which it can be readily separated. Behind, it is 

 connected with the dura mater by processes which pass through the optic foramen 

 and sphenoidal fissure, and with the sheath of the optic nerve. In front it is 

 connected with the periosteum at the margin of the orbit, and sends off a process 

 which assists in forming the palpebral fascia. From its internal surface two 

 processes are given off one to enclose the lachrymal gland, the other to hold the 

 pulley of the Superior oblique muscle in position. 



(2) The sheaths of the muscles give off expansions to the margins of the orbit 

 which limit the action of the muscles. 



1 " On the Oblique Muscles of the Eye in Man and Vertebrate Animals," by John Struthers, M. D. T 

 in Anatomical and Physiological Observations. For a fuller account of the various co-ordinate actions 

 of the muscles of a single eye and of both eyes than our space allows, the reader may be referred to 

 Dr. M. Foster's Text-book of Physiology. 

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