Vein extraction from this image
I would like to direct you to 3 references:


  C. Steger: “Extracting Curvilinear Structures: A Differential
  Geometric Approach”. In B. Buxton, R. Cipolla, eds., “Fourth European
  Conference on Computer Vision”, Lecture Notes in Computer Science,
  Volume 1064, Springer Verlag, pp. 630-641, 1996.
  
  C. Steger: “Extraction of Curved Lines from Images”. In “13th
  International Conference on Pattern Recognition”, Volume II, pp.
  251-255, 1996.
  
  C. Steger: “An Unbiased Detector of Curvilinear Structures”. IEEE
  Transactions on Pattern Analysis and Machine Intelligence, vol. 20,
  no. 2, pp. 113-125, 1998.


In these works, Steger develops a subpixel curvilinear structure extraction algorithm, which is found to work very well for such images. I have also run the algorithm on your images and here you go:

Lines visualized on original image


Lines only


I set the line thickness to 2 for better visualization, normal contours are at sub-pixel level.