1 84 



HISTOLOGY 



charged with nutriment, which had entered the intestinal tissues, and to 

 convey this material back to the blood vessels. Thus the lymphatic 

 vessels were described as tissue spaces, which had elongated and coalesced 

 so as to form tubes bounded by flattened connective tissue cells, and 

 these vessels were thought subsequently to acquire openings into the 

 veins. Opposed to this conception is the idea of Ranvier that the 

 lymphatic vessels are primarily connected with the veins. They grow 

 out from the veins as endothelial sprouts, which form a closed system of 

 endothelial tubes, anastomosing freely with one another, but never with 

 the blood vessels. Thus they are connected with the veins by main trunks 

 comparable with the ducts of glands (Arch. d'Anat. micr., 1897, vol. i, 

 pp. 69-81). Fluids may pass through the thin endothelium almost as 

 readily as through open orifices, so that functionally the distinction does 

 not appear to be fundamental. 



Ranvier's interpretation has been defended by MacCallum, on the basis of histo- 

 logical studies (Arch. f. Anat. u. Physiol., Anat. Abth., 1902, pp. 273-291), and by 

 Miss Sabin, from the injection of the lymphatic vessels in embryos (Amer. Journ. 

 Anat., 1902, vol. i, pp. 367-389). The most convincing evidence in its favor has been 

 supplied by Clark's observations on the growth of the lymphatic vessels in the tails of 

 tadpoles. The tadpoles were anaesthetized with chloretone. The membranous part 



May 16, 



11.30 A.M. 



May 16, 



11.30 P. M. 



May 17, 



II A. M. 



May 18, 



12.30 P. M. 



May 19, 



II A. M. 



May 19, 



IIP. M. 



FIG. 175. SUCCESSIVE STAGES IN THE GROWTH OF A LYMPHATIC VESSEL (lym.) IN THE TAIL OF A TADPOLE 

 (Rana paluslris). Xi3S. (Clark.) b. v., Blood vessel; n., nucleus of the lymphatic vessel. 



of the tail was then examined with immersion lenses, and certain of the lymphatic 

 vessels were drawn. The animals were restored to normal condition and were re-ex- 

 amined at intervals of twelve hours. The growth of a given lymphatic vessel was thus 

 demonstrated, as shown in Fig. 175. Its elongation and enlargement were seen to be 

 independent of the surrounding connective tissue, through which it made its way. 



In some cases a blood corpuscle had escaped into the intercellular spaces. Toward 

 such a corpuscle the lymphatic vessel grew, and having reached it, the corpuscle was 

 taken in by the endothelial cells and transferred to the lumen of the vessel, through 

 which it was seen to travel toward the central vessels. As indicated in Fig. 175, the 

 nuclei of the'living endothelium could be observed, and the multiplication of the endo- 



