THE EYE. 



The canal of Schlemm is often double (fig-. 22) ; it communicates with the spaces between 

 the fibres of the ligamentum pectinatum. and through these with the aqueous chamber of 

 the eye. But, on the other hand, the canal of Schlemm, and the other cavernous spaces in 

 its neighbourhood, are in communication with the reins of the anterior part of the sclerotic, 

 and therefore the aqueous chamber must also through them communicate with the veins. 

 In support of this, it was found by Schwalbe that both the spaces and the veins became 

 filled with coloured fluid when this had been injected into the anterior chamber. Why blood 

 does not find its way into the latter during life is not explained, since no valves have as yet 

 been discovered in these veins or spaces : the reason given being, that greater resistance is 

 offered to its passage here than to its return by the ordinary paths. 



According to Leber, on the other hand, the results obtained by Schwalbe were due to a 

 diffusible colouring matter having been emploj'ed for filling the anterior chamber. Leber 

 affirms that when a non-diffusible one is used it never penetrates into the canal of Schlemm. 

 which is simply a large circular terminal vein, or a collection of two or three plexif orm veins 

 uniting at frequent intervals into one trunk. It is admitted, however, that fluid may pass 

 with extreme readiness from the anterior chamber into these veins. 



The study of the development of the eye shows that the loose tissue in which the spaces 

 of Fontana occur, as well as the endothelium of Descemet's membrane and the membrane 

 itself, belong to a vascular layer of mesoblast which is continuous with the choroidal layer 

 of the embryonic eye, but which as development proceeds, becomes separated from the vascu- 

 lar layer of the choroidal coat (iris and pupillary membrane), owing to the formation of the 

 anterior chamber ; it then comes to form part of the cornea. 



Vessels and nerves. In a state of health the cornea is not provided with 

 blood-vessels, except at the circumference, where the capillaries of the conjunctiva 



Fig. 23. CORNEA OF RABBIT, 



VIEWED ON THE FLAT, 

 SHOWING THE SUBEPITHE- 

 LIAL PLEXUS ; CHLORIDE 

 OF GOLD PREPARATION. 



(Ranvier.) 



n, nerve of fundamental 

 plexus, giving off pencils of 

 fibrils, a, to form the subepi- 

 thelial plexus, e. 



and sclerotic end in 

 loops. Neither are any 

 lymphatic vessels dis- 

 coverable, unless the 

 channels in which the 

 nerves run, and which 

 are lined with flattened 

 cells and are indirectly in 

 connection with the cell- 

 spaces, are to be taken as representing them. The nerves, on the other hand, are 

 very numerous. Derived from the ciliary nerves, they enter the fore part of the 

 sclerotic, and are from forty to forty-five in number (Waldeyer), forming a plexus 

 which surrounds the margin of the cornea (plexus annularis). Continued into the 

 fibrous part of the cornea, partly directly, partly by passing to the adjacent con- 

 junctiva, they retain their medullary sheath for 1 to 2 mm., and then, becoming 

 non-medullated, ramify and form a plexus in the laminated structure, near the 

 anterior surface. From this fundamental plexus branches pass obliquely through the 

 anterior homogeneous lamina, where they divide into pencils of fibrils, whose general 

 direction is towards the centre of the cornea, and which join with one another 

 to form a much finer and closer plexus immediately beneath the epithelium. 

 From this subepithelial plexus fine, varicose, fibrils pass among the epithelium-cells, 

 and form here a terminal ramification which extends almost to the free surface 

 (figs. 23, 24, and 25). 



