XII.] STRUCTURE OF BLOOD-VESSELS. 127 



B. THE LARGER VEINS. 



1. Cut transverse sections of a fairly large vein, as in 

 A. 1. Relatively the muscular element will 

 be found to be less (though varying considerably 

 in different veins) ; the elastic element also to 

 be less, and the white fibrous connective tissue 

 more plentiful. 



2. Attempt to make fluid pass along a vein of some 

 large animal in the wrong direction, in order to 

 shew the action of the valves. Cut open the 

 vein and examine the structure and position of 

 the valves. 



C. CAPILLARIES AND SMALL BLOOD-VESSELS. 



1. Place on a slide with a drop of glycerine the tail 

 of a tadpole which has been hardened in Muller's 

 fluid or in chromic acid '2 p. c. and subsequently 

 treated with alcohol. Brush off the surface epi- 

 thelium or if this is unsuccessful break it up 

 into not too small fragments with needles (in 

 doing so try to scrape off the epithelium). The 

 tissue may be previously stained by placing it 

 for a day in dilute picrocarmine. Note 



a. The network of capillaries ; in some, blood- 

 corpuscles will probably be seen. 



b. The nuclei in the walls of the capillaries, es- 

 pecially at the points of bifurcation. 



c. The branched connective-tissue corpuscles, in 

 places partially surrounding the capillaries, 



