648 MANUAL OF HISTOLOGY. 



fluid may also be driven, and moreover from the subarachnoidal space of 

 the brain. Sclnvalbe, however, states that his " subvaginal " space (sbv) 

 between the internal and external optic sheath does not communicate 

 with the perichoroidal interval. 



The lymphatics of the retina are said to invest the capillaries and veins 

 like sheaths in the adventitial layers, but to pass along side by side with 

 the arteries. They enter the optic nerve, as already stated above. We 

 are still in want of details here. 



Turning now to the anterior lymphatic circulation, we have already 

 remarked that the anterior chamber is regarded as a reservoir for the 

 lymph produced in the iris and ciliary processes. 



In the first place, according to Schivalbe, the lymphatic fluid passes by 

 a series of slits out of the canal of Petit into the posterior, and from 

 thence into the anterior chamber. 



But the supply flowing from Montana's space, through the system of 

 slits in the ligamentum iridis, is of more importance still. This space 

 receives the lymph of the ciliary processes and iris apparently. 



At the border of the membrane of Descemet the opening into the 

 circular canal of Sclilemm takes place, according to Schwalbe. This may 

 be filled with even very slight pressure, and, whether we deny it the 

 nature of a venous vessel or not, certainly does communicate with the 

 venous system. 



This reminds us of a similar arrangement of parts described in the 

 brain by Key and Retzius. 



320. 



Turning now to the accessory structures of the eye, we may pass over 

 the four recti and two oblique muscles as requiring no farther description. 



Many years ago an analogue to the orbital muscle of many mammals 

 was discovered by Mutter in man. This is a greyish-red mass, closing 

 up the fissura orbitalis inferior, consisting of bundles of unstriped muscle 

 fibres, most of which possess elastic tendons. It is supplied by pale, non- 

 medullated nervous filaments from the sphenopalatine ganglion. 



The eyelids, palpebrce are formed of the tarsal cartilages ( 109), 

 enclosed in a thin skin quite devoid of fat. Imbedded in this cartilage 

 a series of peculiar modified sebaceous glands is to be found, known as the 

 Meibomian glands. In man from 30 to 40 of these may be counted in the 

 upper lid, and 20 or less in the lower. In form they are tubes about 

 0*1128 mm. in diameter, with round vesicles attached along the sides. 

 They are somewhat less in length than the tarsal cartilages are in height, 

 and open with contracted mouths on the posterior edge of the free border 

 of the lid. The vesicles of these glands are enveloped in a beautiful net- 

 work of medium-sized capillaries. Their contents, with the exception of 

 a laminated lining of flattened cells in the excretory duct, is the same as 

 that of the sebaceous glands of the skin ( 304). 



The secretion of the Meibomian glands is a thick, whitish-yellow matter 

 containing much fat, and hardening on exposure to the air. It is known 

 as the sebum palpebrale,, and serves to keep the free border of the lid in a 

 greasy condition. 



The orbicularis palpebrarum muscle belongs to the striped class. 

 Associated with it, however, are membranous layers of smooth fibres, 

 forming reticulated fasciculi (H. Mutter), and constituting a ?n. palpe- 

 sup. and inf. 



