706 SPECIAL SENSES 



(25.4 millimeters). The central portion of the upper cartilage is about 

 one-third of an inch (8.5 millimeters) broad, and the corresponding part 

 of the lower cartilage measures about one-sixth of an inch (4.2 milli- 

 meters). At the inner canthus, or angle of the eye, is a small delicate 

 ligament, or tendon, the tendo palpebrarum, which is attached to the 

 lachrymal groove internally, passes outward and divides into two lamellae 

 that are attached to the two tarsal cartilages. At the outer canthus 

 the cartilages are attached to the malar bone by the external tarsal 

 ligament. The tarsal cartilages receive additional support from the 

 palpebral ligament, a fibrous membrane attached to the margin of 

 the orbit and the convex border of the cartilages and lying beneath 

 the orbicularis muscle. This membrane is strongest near the outer 

 angle of the eye. 



On the posterior surface of the tarsal cartilages, partly embedded in 

 them and lying just beneath the conjunctiva, are the Meibomian glands. 

 The structure and uses of these glands have already been described in 

 connection with the physiology of secretion. They produce an oily 

 liquid, which smears the edges of the eyelids and prevents the overflow 

 of tears. 



Muscles that open and close the Eyelids. The corrugator supercilii 

 draws the skin of the forehead downward and inward ; the orbicularis 

 palpebrarum closes the lids ; and the levator palpebrae superioris raises 

 the upper lid. The tensor tarsi, called the muscle of Horner, is a very 

 thin delicate muscle, which is regarded by some anatomists as a deep 

 portion of the orbicularis. Considering this as a distinct muscle, it 

 consists of two delicate slips, which pass from either eyelid behind the 

 lachrymal sac, uniting here to go to its attachment at the posterior por- 

 tion of the lachrymal bone. When this acts with the orbicularis, it 

 compresses the lachrymal sac. 



The orbicularis palpebrarum is a broad thin muscle, closely attached 

 to the skin, surrounding the free margin of the lids and extending a 

 short distance over the bones beyond the margin of the orbit. This 

 muscle may be described as arising from the tendo palpebrarum, the 

 surface of the nasal process of the superior maxillary bone and the in- 

 ternal angular process of the os frontis. From this origin at the inner 

 angle of the eye, its fibres pass elliptically around the fissure of the lids, 

 as indicated above. Its action is to close the lids. In the ordinary 

 moderate contraction of this muscle, only the upper lid is moved ; but 

 in forcible contraction, the lower lid moves slightly and the lids are 

 drawn toward the nose. 



The levator palpebrae superioris is situated within the orbit. It arises 

 from a point a little above and in front of the optic foramen at the apex 



