438 CANAL OF PETIT. 



suspending the vitreous humor, and then puncturing it. In 

 either case the fluid escapes ; but the internal arrangement of 

 the membranes is not visible. 



It is said that when this body is frozen, the portions of fluid 

 in the different cavities, have sometimes been distinguishable 

 from each other, as distinct pieces of ice ; and that their form is 

 that of small wedges, with their edges directed to the crystalline 

 lens as a centre, and their l>ases to the circumference of the 

 vitreous humor. It is also said, that if this body be immersed 

 in a solution of potash, the membranes will become opaque, 

 while the fluid continues transparent ; and that the appearance 

 of the cells, thus exhibited, agrees with the form of the pieces 

 of ice above mentioned. 



This membrane is now generally called the Tunica Hyaloi- 

 dea, from two Greek words which imply a resemblance to glass.* 



Its particular structure is not perfectly understood. Vessels 

 are not generally seen on it in the adult ; but, in the foetus, the 

 artery in the optic nerve, (called the central artery of the retina,) 

 sends a branch through the vitreous humor to the crystalline 

 lens ; and some branches of this artery are to be seen on the 

 tunica hyaloidea.f 



The reflection of the hyaloid membrane over this branch of 

 the artery in its course from behind forwards through the 

 centre of the vitreous body, forms a passage for the vessel, 

 called canalis hyaloideus. 



Throughout the greatest part of its extent, it appears to con- 

 sist of but one lamen ; but at the front part, near the ciliary 

 processes, there are two lamina. The internal, or posterior, 

 seems to be a portion of the proper tunica hyaloidea, and 

 passes behind the crystalline lens. The external, or anterior, 

 passes over and before the lens ; or at least is attached to the 



* Discovered by Fallopius in 1584. 



f It is also believed that vessels pass from the ciliary processes of the choroid 

 to the ciliary folds (zona eiliaris) of the hyaloid tunic, between which the cho- 

 roid processes are embedded. The hyaloid tunic must be very vascular, though 

 usually containing only white blood, for in calves and sheep driven with blows 

 to the shambles, the vitreous humor is often tinged with blood. P. 



