of Animal Bodies. 305 



Thus in the mesentery of a living frog spread upon glass, a mi- 

 croscope with a magnifying power of 300 exhibits branches of 

 veins measuring from 0,014 to 0,028 (millim.) in diameter, 

 which are perfectly transparent, and can only be distinguished 

 at their edges, being as simple as if they were merely covered 

 with a pellicle of pure water.'' 



We now come to a part of Treviranus's treatise, which is of 

 peculiar importance, and which I dwell on with more than ordi- 

 nary pleasure, because it contains the fullest and most satisfac- 

 tory proofs of the correctness of the opinion, that the lymphatics 

 are the veins of the white tissues, an opinion which I was the 

 first to promulgate. 



" In some parts of the body I observed circumstances attend- 

 ing these minute and simple vessels, which render it probable 

 that they terminate in tubes of such a degree of fineness that 

 they are incapable of admitting the globules of the blood, and 

 consequently contain a serous fluid only. This is the case with 

 those branches of the central artery of the retina, that proceed 

 to the posterior surface and lateral edges of the capsule of the 

 lens. Even at their origin from the central artery, these vessels 

 become at once incapable of carrying red blood, the very pre- 

 sence of which in these parts would be destructive to the trans- 

 parency so essential to vision." 



Treviranus then proceeds to detail minutely the situations in 

 the eye, lungs, and other parts where he has observed these pro- 

 longations of the arteries containing none of the coloured blood ; 

 and indeed, from their diameter being much smaller than that 

 of the globules, it is evident that they are incapable of admitting 

 them. He then examines their structure, as compared with 

 that of the minute lymphatics, and endeavours, I think, success- 

 fully to solve the long-disputed question as to the origin of the 

 lymphatics, and their connexion with other portions of the cir- 

 culating system. In the tortoise and cold-blooded reptiles, it is 

 easy to make out with the aid of the microscope the continuity 

 of the roots of the lymphatics, with the elementary cylinders of 

 the cellular membrane. The latter gradually coalesce, and as- 

 sume new appearances as they are about to become lymphatics. 

 Now we have already seen, that extremely minute ramifications 

 of the arteries are the bearers of the serous fluid to the white 



