692 SPECIAL HISTOLOGY. 



"laries, as they become wider, present more closely placed nuclei, and 

 are then invested externally with a structureless t. adventitia, and soli- 

 tary muscle-cells, whence, when they have reached the diameter of 

 0-007 of a line, they already exhibit the aspect of the finest arteries 

 (Fig. 287, 1). Afterwards, the nuclei seem to be replaced by epithe- 

 lium cells, whilst the capillary membrane either ceases, or is continuous 

 with the elastic inner membrane. The venous transitionary vessels are 

 less characteristic for a greater length. The first thing that is super- 

 added, on this side, to the capillary membrane, is an external, homo- 

 geneous, nucleated layer, which may be regarded as a sort of connec- 

 tive tissue, and whilst the nuclei of the capillary vessels become more 

 closely approximated, gradually coalesces with their membranes. In 

 vessels of 0-01 of a line, 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 appear 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 t. 



intima. 



i 



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 vessels 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, 

 0-0009 of a line) 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 oppo- 

 sition to this, Briicke and Gerlach remark, that the cornea! vessels ter- 

 minate in true loops, and that it would thence appear that Hyrtl's state- 

 ments 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 previous 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 communicated with the canals of the true 



