The Normal Eyelid

Figure 5.8 is a diagram of the normal eyelid in cross-section. The lids contain two antagonistic voluntary muscles: the more superficial orbicu-laris oculi, supplied by the seventh cranial nerve, which closes the eye, and the tendon of the levator palpebrae superioris, supplied by the third cranial nerve, which opens the eye. We must not forget that there is also some smooth muscle in the upper and lower eyelids, which has clinical importance apart from its influence on facial expression when the subject is under stress. Loss of tone in this muscle accounts for the slight ptosis seen in Horner's syndrome; increased tone is seen in thyrotoxic eye disease. These muscles (that in the upper lid is known as Muller's muscle) are attached to the skeleton of the lid, which is the tarsal plate, a plate of fibrous tissue (not cartilage) that contains the meibomian glands.

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