CIRCULATORY CHANNELS 



161 



int. 



med. 



appear according to McClure in close proximity to some of the early 

 blood vessels, which latter they subsequently crowd out of existence as 

 active blood channels. According to Miss Sabin, lymph channels are 

 developed as outgrowths of veins in the throat and in the inguinal 

 region. 



Like the blood channels, the lymph channels are lined with a flat 

 endothelium. Unlike the blood vessels, they do not acquire the thick 

 muscular and elastic walls, 

 because there is no great 

 pumping pressure on them 

 and the weight of the lymph 

 is not so great. Where any 

 approach to size is found, the 

 strengthening of the wall is 

 arranged as it is in a smaller 

 vein. 



The lymph channels com- 

 municate with the blood chan- 

 nels directly at a few points 

 where the lymph, bearing food 

 materials, pours into the 

 blood. It also is connected 

 with the blood space by tem- 

 porary clefts through which 

 plasma and lymph cells can 

 pass, but not the red blood 

 corpuscle. 



The lymph vessels possess 



Valves Which are USUally FlG 14S ._ Port i " n ' O f a transection of an artery 

 double and are CVaginated f rom man - *'' intima ; med., media ; ad., ad- 

 folds Of the Walls. They also venti^^CFrom "SXOHR'S Text-book of 



have regions where the pres- 

 sure of muscles on enlarged parts of the channels causes, with the aid 

 of the valves, a slow and irregular circulation. 



Technic. The technic is usually the simplest. Flemming's fluid 

 and paraffin sections are the best to use for the general kinds of blood 

 vessels. Silver nitrate may often be used for the demonstration of the 

 epithelial layers on the outer and inner surfaces. 



LITERATURE 



Read concerning the mammalian blood vessels in the medical histologies. 

 ARGAND. "Sur la Structure des Arteries chez les Oiseaux,'' C. R. Ass. Anat. Sess., Vol. VI, 

 p. 90, 1904. 



M 



