422 THE SKIN AND THE SENSE-ORGANS. 



These branched form a capillary network (Fig-. 260 I) with 

 elongated meshes, formed by the capillaries anastomosing 1 at acute 

 angles. The capillary network is more dense towards the middle 

 of the proximal surface than in other parts. 



The veins arising from this network are three in number (Fig. 

 260 III) ; two of these accompany the arteries from their origin, 

 and form a somewhat similar circle around the lens, while the third 

 passes backwards along the ventral surface of the vitreous body to 

 the papilla nervi opfici. The nasal vein, however, takes a more 

 proximal course than the corresponding artery, the branches of 

 which it crosses ; consequently the venous ring is not so perfect as 

 the arterial. The nasal vein is larger and the temporal vein smaller 

 than the corresponding arteries. 



The ventral vein is formed near the papilla nervi opfici by the 

 union of two smaller branches. The capillary system of these 

 vessels has the usual structure of capillaries, the cells being united 

 by cement-substance (Zimmermann). 



The blood-vessels of the vitreous body are accompanied by 

 lymphatics; according to Iwanoff they completely enclose the 

 capillaries : Zimmerman contradicts this view, as he has been un- 

 able to find lymphatics on that side of the capillaries directed 

 towards the vitreous body. 



B. Appendages of the eye. 



The appendages of the eye are the eye-muscles (see pp. 55-59), 

 the eyelids, the Harderian gland, and the lachrymal duct. 



a. The eyelids are two in number, an upper and a lower. The 

 upper eyelid is intimately attached to the eyeball and follows the 

 movements of that organ. 



The lower eyelid (membrana nictitans) is much larger than 

 the upper and has the same functions as the lower eyelid of higher 

 vertebrates. It forms a transparent covering for the eyeball, and 

 is raised by a special muscle (see p. 58) ; functionally it takes the 

 place of both eyelids of higher vertebrates. 



The lower eyelid is a prolongation of the skin, but has only a 

 few pigment-cells, except at its free margin, and no serous glands. 

 Mucous glands are found in two or three rows, closely applied to 

 one another, on the superficial surface of the lid ; on the deeper surface 

 they are wanting. The stroma of the lid, like the cutis, is of con- 

 nective-tissue. 



Nerve-fibres can be traced in all directions through the substance 



