DISSECTION OF THE HEAD AND NECK. 179 



portion, including that covering the caruncula and membrana nictitans, 

 consists of a stratified epithelium and a papillated layer of vascular 

 subepithelial connective-tissue containing small mucous glands. The 

 ocular portion where it covers the sclerotic resembles the preceding in 

 structure, but is thinner, nonpapillated, and less vascular ; where it 

 passes over the cornea, it consists of the epithelium only, which is 

 generally enumerated as one of the constituent layers of the cornea 

 itself, being termed its anterior epithelium. Through the puncta 

 lachrymalia the conjunctival epithelium is continuous with that lining 

 the lachrymal canals ; and at the free margin of the lids it is continuous 

 with the epidermis. 



Structure of the Eyelids. — Each lid is composed of the following 

 parts : a layer of skin outwardly, the palpebral conjunctiva inwardly, 

 a portion of the orbicularis palpebrarum muscle, the palpebral tendon, 

 the tarsal cartilage, vessels, and nerves. 



Besides these, there is found in the upper lid the tendon of a special 

 muscle — the levator palpebrse superioris. Of these, the skin and con- 

 junctiva have already been sufficiently noticed. 



The Orbicularis Palpebrarum (Fig. 19, page 161). This muscle will 

 be exposed by removing the layer of skin from the eyelids, and from 

 around the orbital rim for the breadth of an inch. The fibres o*f the 

 muscle are closely adherent to the skin of the eyelids, and have a 

 circular or elliptical disposition' around the palpebral fissure. Below the 

 nasal canthus a number of the fibres are inserted by a slender tendon 

 into the lachrymal tubercle on the bone of the same name, but for the 

 most part the fibres are without bony attachments. Above the orbit the 

 muscle is confounded with some thin fibres — the corrugator supercilii — 

 which wrinkle the overlying skin. 



Action. — The orbicularis muscle closes the eye by approximating the 

 free edge of the eyelids, and in this action the upper lid has a much 

 wider range of movement than the lower. 



The Palpebral Tendon. This is a fibrous layer which will be 

 exposed by removing the orbicularis muscle. At the free edge of 

 the eyelid it is margined by the tarsal cartilage, while by its opposite 

 border it is fixed to the rim of the orbit. 



The Tarsal Cartilage. This is a slender rod of elastic cartilage 

 imbedded in the free edge of each eyelid. The meibomian glands lie in 

 grooves on its ocular surface. It prevents the margins of the lids from 

 being drawn, or puckered, when the orbicular muscle contracts. 



The Levator Palpebral Superioris. This muscle is found in the 

 upper eyelid only. The fleshy portion of the muscle is a slender flat 

 fasciculus which takes origin at the back of the orbit (Fig. 25, page 215) 

 above the optic foramen, where it will subsequently be exposed along 

 with the other contents of the cavity. Passing along the eyeball and 



