THE ORGANS OF SPECIAL SENSE 



575 



tubules. The duct is lined with stratified squamous epithehum. 

 The tubules resemble those of the sebaceous glands. Between the 

 tarsus and the skin are the muscular structures of the eyehd in 

 which both smooth and striated muscle are found. 



Blood-vessels. — -Two main arteries pass to the eyelid, one at 

 each angle, and unite to form an arch, the tarsal arch, along the 

 margin of the lid. A second arch, the external tarsal arch, is formed 

 along the upper margin of the tarsus. From these arches are given 

 off capillary networks w^hich supply the structures of the lid. 



Lymphatics. — These form two anastomosing plexuses, one ante- 

 rior, the other posterior to the tarsus. 



Nerves. — The nerves form plexuses in the substance of the lid. 

 From these, terminal fibrils pass to the various structures of the lid. 

 Many of the fibres end freely in fine networks around the tarsal 

 glands, upon the blood-vessels, and in the epithelium of the conjunc- 

 tiva. Other fibres terminate in end-bulbs which are especially nu- 

 merous at the margin of the lid. 



Development of the Eye 



The eyes begin their development very early in embryonic life. As optic 

 depressions they are visible even before the closure of the medullary groove. 



Fore-brain vesic'.e 



Lens area--r=i% 



Optic vesicle 



Surface ectoderm 



^Optic vesicle 



FiG_. 381. — Section through head of chick of two days' incubation. (Duval.) The 

 formation of the optic vesicle and stalk appears to be somewhat more advanced on the 

 left than on the right. 



As a result of the closure of this groove, the optic depressions are transformed 

 into the optic vesicles. The connection between vesicle and brain now becomes 

 narrowed so that the two are connected only by the thin optic stalk. The 

 surface of the optic vesicle becomes firmly adherent to the epidermis and as a 



