XII.] STRUCTURE OF BLOOD-VESSELS. 129 



earlier stages of the action of acetic 

 acid, the outlines of the muscle fibres, 

 as well as of their nuclei, will be dis- 

 tinctly seen. 



(7) The thin inner elastic layer, internal to 

 the muscular fibres, seen as wrinkled 

 longitudinal markings. 



(S) The oval nuclei of the lining epithelium 

 arranged longitudinally. 



c. In the small veins, appearances similar to 6, 

 consistent with the less development of mus- 

 cular and elastic tissue. 



A piece of fresh pia mater may be stained with 

 aqueous haematoxylin to bring out more clearly some 

 points of the above ; the following will, however, be 

 found a better method. Place a fresh brain, with 

 the dura mater at least in part removed, in potas- 

 sium bichromate 1 p. c. for two or three days (or 

 longer), then wash it with water, and place it in 

 75 p. c. spirit for half an hour to twenty-four as 

 convenient, cut off then a small piece of pia mater, 

 stain with lisematoxylin, or picrocarmine, and mount. 

 Care should be taken to drag the membrane as little 

 as possible, and to avoid creases. 



4. Destroy the brain and spinal cord of a frog, lay 

 bare the heart, cut through the whole length of 

 the muscles of the abdomen a little on one side 

 of the epigastric vein. Cut off the ventricle 

 and cut across the venous sinus (cp. Less. XI. 

 B. II.), sop up with a sponge the blood which 

 L. 9 



