ORGANS OF SENSE. 165 



The lachrymal sac is the upper extremity or oval cul de sac of the nasal 

 duct, distinguished from it externally only by a small constriction, and 

 internally b3'' a semilunar fold. It is situated opposite to the middle meatus 

 of the nose, receives the lachrymal secretions from the small ducts, and 

 transmits them to the nose. It is usually filled with adhesive mucus. 



The nasal duct leads obliquely downwards and outwards from the sac, and 

 opens into the anterior part of the outer side of the roof of the lower meatus 

 of the nose under cover of the inferior turbinated bone. It is about three 

 fourths of an inch long, ii little curved, and separated from the antrum 

 by a thin but strong bony lamella. It is a fibro-mucous duct, inclosed 

 in and rather loosely adherent to the bony canal, formed by the maxillary, 

 unguis, and inferior spongy bones. 



4. Muscles of the Eye. The orbit of the eye contains seven muscles, 

 the interstices of which are filled with a quantity of soft adipose substance. 

 These muscles are the levator palpebrae, four recti, and two oblique. All 

 these except the inferior oblique arise near or around the apex of the cavity, ; 

 and thence diverge to their respective insertions. These are situated above. 

 the optic nerve, the levator palpebral, superior oblique, and superior rectus ; 

 two are beneath it, the inferior rectus and inferior oblique ; and one is at 

 either side, the internal and external rectus. 



The levator palpebrm supenoris is one of the largest and highest muscle.'- 

 in this region. It arises by radiated tendinous fibres from the upper or 

 anterior border of the foramen opticum, and from tlie fibrous sheath of the 

 optic nerve. It passes forwards in the axis of the orbit, becomes broad, 

 thin, and fleshy, anteriorly bends downwards in front of the eye, and end.s 

 in a thin membranous expansion, which is inserted into the convex border 

 of the superior tarsal cartilage, as also into the convexity of the superior 

 palpebral sinus of the conjunctiva, behind or beneath the broad ligament of 

 the tarsus. It serves to elevate the upper eyelid, also to retract its cartilage 

 beneath the edge of the orbit. A branch of the third or motor nerve is 

 distributed to it, which, with the muscle when paralyzed, permits the uppei' 

 eyelid to droop downwards in front of the eye. 



The remaining six muscles are jDroper to the eye-ball. Four are called 

 straight, and two oblique. By their varied combinations, all the motions 

 of the eye-ball are performed. The recti muscles are by no means straight, 

 since all arise from the apex of the orbit, and diverging as they pass 

 forwards, inclose a pyramidal space embracing the greater part of the 

 globe of the eye, and bending a little around the fore part of the eye to 

 reach their insertion. Each rectus muscle, then, represents a curve with its 

 concavity towards the eye-ball. They arc each of a triangular form, the 

 apex behind, the base before. All terminate in front by thin tendons which 

 extend to within a few lines of the circumference of the cornea. They are 

 connected together by a cellulo-fibrous tissue named ocular fascia. The 

 names assigned to the diffei'cnt recti muscles are rectus superior^ or levatw 

 oculi ; rectus internus^ or adductor oculi; rectus inferior^ or depressor oculi ; 

 and rectus externus, or abductor oculi. 



The oblique muscles are two in number, the superior and the inferior. 



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