1078 SPECIAL SENSE ORGANS 



embedded. One tarsal border is free, viz., toward the edge of the lid, the other 

 is attached; the former is straight, while the latter is convex, especially in the 

 upper lid. 



The length of each tarsus is about twenty millimetres. Its breadth is greatest in the middle 

 of the lid, and becomes gi-adually smaller toward each angle, where the tarsi are joined to the 

 lateral raphe and medial palpebral ligament. The breadth of the upper tarsus (10 mm.) is 

 about twice that of the lower. The thickness of each is greatest, and its texture closest, at the 

 middle of its length, thinning off toward the angles of the eye and toward both borders. Into 

 the superior anterior border of the upper tarsus the lower layer of the levator expansion is 

 attached, consisting of smooth muscle-fibres constituting the superior tarsal muscle of Miiller. 

 In like manner, at the inferior border of the lower tarsus, bundles of smooth muscle-fibre are 

 inserted (the inferior tarsal muscle of Miiller), developed in what has been regarded as part 

 of the extension of the sheath of the inferior rectus. 



The palpebral conjunctiva is firmly adherent to the posterior aspect of the 

 tarsus; but in the orbital part of the lid loose subconjunctival tissue intervenes 

 between it and Mliller's tarsal muscle. Lymphoid tissue occurs in the substance 

 of the conjunctiva, especially in its orbital division. Near the upper fornix, the 

 conjunctiva receives expansions of the tendon of the levator palpebrae and of the 

 sheath of the superior rectus, and, at the lower fornix, of the sheath of the inferior 

 rectus. The surface of the tarsal conjunctiva shows small elevations or papillae 

 everywhere; but these are particularly well marked over the attached border of 

 the tarsus. 



Glands of the eyelids. — From its manner of formation the eyelid may be 

 regarded as consisting of two thicknesses of skin, the posterior having been 

 doubled back upon the anterior at the edge of the lid; thus the epidermis and 

 corium of the skin proper are represented respectively by the conjunctiva (epi- 

 thelium) and tarsus of the inner thickness. At the free border of the lid, accord- 

 ingly, we find glands corresponding to the sebaceous and sweat-glands of the skin, 

 viz., large sebaceous glands of the cilia (Zeiss's glands) and the ciliary glands of 

 Moll, which are modified sweat-glands. Again, in the inner skin-thickness of. 

 the lid, the tarsal (Meibomian) glands are sebaceous. 



Acino-tubular mucous glands occur at the attached border of the tarsus (Krause's or 

 Waldeyer's glands), and similar glands also occur at the fornix, and are especially abundant 

 near the outer angle of the upper lid, close to the efferent ducts of the lacrimal gland; from their 

 structure and the character of their secretion, these acinous or acino-tubular glands have been 

 termed by Henle 'accessory lacrimal glands.' Other simple tubular glands (Henle), formed 

 merely by the depressions between the papillae, are best developed in the medial and lateral 

 fourths of the tarsal conjunctiva of both lids. 



Blood-vessels. — The arteries run in the central connective tissue of the lids, mainly in 

 the form of arches near the borders of the tarsus, from which twigs go to the different pal- 

 pebral tissues. They are supplied by the lacrimal and palpebral branches of the ophthalmic, 

 and by small branches derived from the temporal artery. The veins are more numerous and 

 largei than the arteries, and form a close plexus beneath each fornix. They empty themselves 

 into the veins of the face at the medial, and into the orbital veins at the lateral angle of the eye. 



The lymphatic vessels of the lids are numerous, and are j^rincipally situated in the con- 

 junctiva. Lymph-spaces also surround the folli(;les of the tarsal glands. The palpebral 

 lymphatic vessels from the lateral tlireo-fourtlis of the lid pass through the anterior auricular 

 and jiarotid nodes; those from the medial fourth of the lower lid go to the facial and subma.xil- 

 lary lyiiipliatic nodes. 



Nerves. — (a) Sensory. The upper lid is chiefly supplied by branches of the supraorbital 

 and supratrochlear nerves, the lower lid by one or two branches of tlie infraorbital. At the 

 medial angle the infratrochlear nerve also aids in the supply, and, at the lateral angle, the 

 lacrimal. (6) Motor. The palpebral part of the orbicularis is supplied by branches of the 

 facial nerve, which mainly enter it near the lateral angle. The tarsal muscles are suppUed 

 by the sympathetic nervous system. 



The medial palpebral ligament has been referred to previously. Arising from 

 the frontal jjroccss of the niuxilla, it extends laterally over the front wall of the 

 lacrimal sac, bends round the lateral wall of the sac, and then passes l)ackward to 

 the p(jsterior crest on the lacrimal ))one. It is thus U-shajied, having its limbs 

 anterior and posterior, embracing the huTimal sac; the anterior limb lies immedi- 

 ately b(;neath the skin, and is visible in the living. The })alpebral fil)res of the 

 orbicularis are in.serted into the anterior surface of l)oth limbs, those attached to 

 the posterior limb constituting the pars lacrimalis of the orl)icularis palpebrarum 

 (Horner's muscle)- The lateral palpebral raphe is m(>rely a stronger development 

 of connective tissue in the orbicularis. Both ligaments are connected with the 

 tar.si as alreadv mentioned. 



