844 



THE ORGANS OF SPECIAL SENSE. 



about an inch in length. They are placed one in each lid, contributing to their 

 form and support. 



The superior, the larger, is of a semilunar form, about one-third of an inch in 

 breadth at the centre, and becoming gradually narrowed at each extremity. To 



the anterior surface of this plate the 

 aponeurosis of the Levator palpebrse is 

 attached. 



The inferior tarsal plate, the smaller, 

 is thinner and of an elliptical form. 



The free or ciliary margin of these 

 plates is thick, and presents a perfectly 

 straight edge. The attached or orbital 

 margin is connected to the circumference 

 of the orbit by the fibrous membrane of 

 the lids, with which it is continuous. The 

 outer angle of each plate is attached to 

 the malar bone by the external tarsal 

 ligament. The inner angles of the two 

 plates terminate at the commencement 

 of the lacus lacrimalis ; they are attached 

 to the nasal process of the superior max- 

 illa by the internal tarsal ligament or 

 tendo oculi. 



The palpebral ligaments are mem- 

 branous expansions situated one in each 

 lid, and are attached marginally to the 

 edge of the orbit, where they are con- 

 tinuous with the periosteum: The supe- 

 rior ligament blends with the tendon of 

 the Levator palpebrae, the inferior with 

 the inferior tarsal plate. Externally the 

 two ligaments fuse to form the external 

 tarsal ligament, just referred to ; inter- 

 nally they are much thinner and, be- 

 coming separated from the internal tarsal 

 ligament, are fixed to the lachrymal bone 

 immediately behind the lachrymal sac. 

 Together, the ligaments form an incom- 

 plete septum, the septum orbitale, which 

 is perforated by the vessels and nerves 

 which pass from the orbital cavity to the 

 face and scalp. 



The Meibomian glands (Fig. 453) are situated upon the inner surface of the 

 eyelids between the tarsal plates and conjunctiva, and may be distinctly seen 

 through the mucous membrane on everting the eyelids, presenting the appear- 

 ance of parallel strings of pearls. They are about thirty in number in the upper 

 eyelid, and somewhat fewer in the lower. They are imbedded in grooves in the 

 inner surface of the tarsal plates, and correspond in length with the breadth of 

 each plate ; they are, consequently, longer in the upper than in the lower eyelid. 

 Their ducts open on the free margin of the lids by minute foramina, which cor- 

 respond in number to the follicles. The use of their secretion is to prevent adhe- 

 sions of the lids. 



Structure of the Meibomian Grlands. These glands are a variety of the cuta- 

 neous sebaceous glands, each consisting of a single straight tube or follicle, having 

 a csecal termination, and with numerous small secondary follicles opening into it. 

 The tubes consist of basement-membrane, lined at the mouths of the tubes by 

 stratified epithelium ; the deeper parts of the tubes and the secondary follicles are 



FIG. 452. Vertical section through the upper eye- 

 lid. (After Waldeyer.) a. Skin. b. Orbicularis palpe- 

 brarum. b'. Marginal fasciculus of orbicularis (ci\- 

 iary bundle) c. Levator palpebrae. d. Conjunctiva. 

 e. Tarsal plate. /. Meibomian gland, g. Sebaceous 

 gland. /(. Eyelashes. /. Small hairs of skin. j. Sweat- 

 glands, k. Posterior tar.sal glands. 



