312 SPECIAL HISTOLOGY. 



communicated with the canals of the true capillaries, could 

 not be determined, and I should not, therefore, at present 

 definitively declare them to be pervious parts of the vascular 

 system ; whilst I have not the least hesitation, nevertheless, in 

 referring them to that system, for although they may have no 

 canal, they scarcely admit of any other possible interpretation, 

 than as being derived from the vascular plexus, which covers 

 almost the entire cornea in the child at birth, and as being 

 obliterated capillaries. Should these corneal elements not 

 turn out to be vasa serosa, I am acquainted, in the adult, with 

 no situation in which such vessels exist, whilst vessels con- 

 veying plasma are everywhere present during the development 

 of the capillaries, as a preliminary phenomenon (via 1 , infra), and 

 it is therefore intelligible that, even at a subsequent period, 

 scattered vessels of the kind should occasionally be met with, 

 as in the brain of the Calf, according to Henle ; or may per- 

 haps exist in great quantity, just as in the distribution of the 

 nerves, the terminations often retain the embryonic character.] 



3.— OF THE LYMPHATICS. 

 §218. 



The lymphatics, except in their contents, correspond so 

 closely with the veins, that a short exposition of their structure 

 will suffice. 



Thecapillary lymphatics, which, in the three situations in which 

 they have hitherto been seen with certainty — in the small intes- 

 tine, the tail of the Tadpole, and the mucous membrane of the 

 trachea — commence partly with free prolongation, in part in net- 

 works. I have, in a single instance in the tracheal mucous mem- 

 brane in Man, had an opportunity of investigation, and they 

 were found to consist of a delicate structureless wall, without 

 distinct nuclei after the addition of soda, and having a diameter 

 of 0-003— 0005— 0-01'" (fig. 235). The same structure is pre- 

 sented in the simple lacteals of the intestinal villi in Mammalia, 

 except that these measure 0012 — 0-026'", and have a somewhat 

 thicker wall. The lymphatics discovered by me in the tail of 

 the Tadpole (fig. 288), on the contrary, correspond entirely 



