THE BLOOD-VESSELS. 311 



that is superadded, on this side, to the capillary membrane, is 

 an external, homogeneous, nucleated layer, which may be 

 regarded as a sort of connective tissue, and whilst the nuclei 

 of the capillary vessels become more closely approximated, 

 gradually coalesces with their membranes. In vessels of 001'", 

 the internal nuclei have become so numerous, as clearly to 

 represent the epithelium, and as at this time also, the external 

 layer has likewise received the addition of a nucleated lamina 

 — the t. adventitia — the now distinctly laminated vessel 

 (fig. 280) may be termed a vein. It would consequently ap- 

 pear that the capillaries are transformed into the larger vessels 

 by the addition of layers on the exterior and interior, whilst 

 their proper membrane coalesces with these layers, and is 

 probably continuous with the fibrous layer of the /. intima. 



[Besides the finest capillaries, which, however, always admit 

 of the passage of the very flexible blood-corpuscles, the 

 older anatomists have admitted the existence of still finer ves- 

 sels — the so -termed vasa serosa — which no longer allow of 

 the passage of blood, but only of its plasma, — a notion which 

 has been abandoned by most modern authors. Hyrtl alone 

 thinks, that it is necessary to admit of the existence of vessels 

 of this kind in the cornea, because the vessels at its border 

 escape the sight without passing into veins, and are too small 

 (in Man, when injected, 00009"') to be capable of conveying 

 blood-corpuscles. He thinks, that still further on they are 

 continued into vasa serosa, and probably are connected with 

 the, as yet undemonstrated, lymphatics. In opposition to 

 this, Briicke and Gerlach remark, that the corneal vessels 

 terminate in true loops, and that it would thence appear that 

 Hyrtl's statements are based upon incomplete injections. I 

 am able, however, to state that something corresponding with 

 the vasa serosa of authors does actually exist in the cornea, 

 having noticed that, in the Dog, fine and the finest filaments 

 are continued still further inwards, from the terminal loops 

 pervious to the blood-corpuscles, which occur in that animal, as 

 in all others, at the margin of the cornea ; which filamentary 

 prolongations were connected in a reticular manner, and were 

 usually slightly dilated at the points of junction. Whether 

 these filaments were hollow, and had any contents, and directly 



