THE ORGANS OF SPECIiVL SENSE 573 



cornea first form a plexus in the sclera — the plexus annularis — which 

 encircles the cornea. From this, branches pierce the substantia 

 propria of the cornea, where they form four corneal plexuses, one in 

 the posterior part of the substantia propria, a second just beneath 

 the anterior elastic membrane, a third sub-epithelial, and a fourth 

 intra-epithelial. The fibres of the last named are extremely delicate 

 and terminate freely between the epithelial cells. Krause describes 

 end-bulbs as occurring in the substantia propria near the margin 

 of the sclera, while according to Dogiel some of the fibres are con- 

 nected with end-plates. 



The Lacrymal Apparatus 



The lacrymal apparatus of each eye consists of the gland, its 

 excretory ducts, the lacrymal canal, the lacrymal sac, and the nasal 

 duct. 



The lacrymal gland is a compound tubular gland consisting of 

 two main lobes. Its structure corresponds in general to that of a 

 serous gland. The excretory ducts are lined with a two-layered col- 

 umnar epithelium which becomes simple columnar in the smaller 

 ducts. The alveoli are lined with irregularly cuboidal serous cells, 

 which rest upon a basement membrane beneath which is a richly 

 elastic interstitial tissue. 



The lacrymal canals have a stratified squamous epithelial lining. 

 This rests upon a basement membrane beneath which is the stroma 

 containing many elastic fibres. External to the connective tissue are 

 some longitudinal muscle fibres. 



The lacrymal sac is lined with a two-layered stratified or pseudo- 

 stratified columnar epithelium resting upon a basement membrane. 

 The stroma contains much diffuse lymphatic tissue. 



The nasal duel has walls similar in structure to those of the lacry- 

 mal sac. In the case of both sac and duct the walls abut against 

 periosteum, a dense vascular plexus being interposed. 



The blood-vessels, lymphatics, and nerves of the lacrymal gland 

 have a distribution similar to those of other serous glands. 



The Eyelid 



The eyelid consists of an outer skin layer, an inner conjunctival 

 layer, and a middle connective-tissue layer. 



The epidermis is thin and the papillae of the derma are low. Small 

 sebaceous glands, sweat glands, and fine hairs are present. 



