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 by 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, a 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. Muscuizs or tut 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 palpebre, 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 situ- 
ated above the optic nerve, the levator palpebre, 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 palpebre superioris is one of the largest and highest muscles 
in this region. It arises by radiated tendinous fibres from the upper or 
anterior border of the foramen opticum, and from the 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 ends 
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 paralysed, permits the upper 
eyelid to droop downwards in front of the eye. 
The remaining six muscles are proper 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 rectt 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 are 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 different recti muscles are rectus superior, or levator 
oculi; rectus internus, or adductor oculi; rectus inferior, or depressor 
oculi ; and rectus externus, or abductor ocult. 
~The oblique muscles are two in number, the superior ahd the inferior. 
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