908 THE ORGANS OF SENSE. 



The tarsal plates (cartilages} l are two thin elongated plates of dense connect- 

 ive tissue about an inch in length. They are placed one in each lid, contribut- 

 ing 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. Into 

 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 palpebral 

 or tarsal ligament. The inner angles of the two plates terminate at the com- 

 mencement of the lacus lachrymalis, being fixed to the margins of the orbit by 

 the tendo oculi. 



The tarsal ligament, or fibrous membrane of the lids, is a layer of fibrous 

 membrane beneath the Orbicularis, attached marginally to the edge of the orbit, 

 where it becomes continuous with the periosteum, and centrally to the tarsal 

 plate, near its ciliary margin, with the tissue of which it is continuous. It is 

 thickest and densest at the outer part of the orbit. Upon its under surface is a 

 layer of unstriped muscle, which in the upper lid passes from the aponeurosis of 

 the Levator palpebrse muscle to the tarsal plate. This ligament serves to support 

 the eyelids, and retains the tarsal plates in their position. 



The Meibomian glands (Fig. 537) 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 peculiar parallel arrangement of these 

 glands, side by side, forms a smooth layer adapted to the surface of the globe, 

 over which they constantly glide. The use of their secretion is to prevent adhe- 

 sions of the lids. 



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

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

 ing a caecal termination, and with numerous small secondary follicles opening 

 into it. The tubes consist of basement membrane, covered by a layer of scaly 

 epithelium ; the secondary follicles are lined by a layer of polyhedral cells con- 

 tinuous with the cells of the tube. The remainder of the follicle is filled with 

 large polyhedral cells charged with fat. They are thus identical in structure 

 with the sebaceous glands. 



The conjunctiva is the mucous membrane of the eye. It lines the inner surface 

 of the eyelids, and is reflected over the fore part of the sclerotic and cornea. In 

 each of these situations its structure presents some peculiarities. 



The palpebral portion of the conjunctiva is thick, opaque, highly vascular, and 

 covered with numerous papillae, its deeper parts presenting a considerable amount 

 of lymphoid tissue. At the margin of the lids it becomes continuous with the 

 lining membrane of the ducts of the Meibomian glands, and, through the lachrymal 

 canals, with the lining membrane of the lachrymal sac and nasal duct. At the 

 outer angle of the upper lid it may be traced along the lachrymal ducts into the 

 lachrymal gland, and at the inner angle of the eye it forms a semilunar fold, 

 the plica semilunaris. The folds formed by the reflection of the conjunctiva from 

 the lids on to the eye are called the superior and inferior palpebral folds, the 



'Recent observations have proved that the so-cnlled "tarsal cartilages" do not contain any carti- 

 lage-cells, and that the name is a misnomer. 



