THE FROG 95 



branches since these often fail of satisfactory injection. 

 Such a preparation may be pinned out on a plate of wax and 

 exhibited in a jar of alcohol. 



3. The veins. Frogs will be needed especially for the 

 study of the venous system, but the veins need not be in- 

 jected artificially since the blood affords a natural injection 

 of the veins by collecting in them after death, leaving the 

 arteries empty. The specimens should be killed with ether 

 and preserved in weak formalin (2 per cent, or 3 per cent.) ; 

 an incision should be made through the ventral wall of the 

 abdomen to allow the formalin to enter the body cavity. 

 They should be rinsed for a few hours in water before being 

 used for dissection. 



4. The capillaries, and their relation to arteries and veins, 

 are best demonstrated in the web of the foot of a living frog. 

 For this purpose a young frog is best, since the web is 

 thinner and contains less pigment than in an older specimen. 

 Fasten the live frog to a small piece of thin board by means 

 of a cloth jacket which will not interfere with the circula- 

 tion; spread out the toes of a hind foot so that the web will 

 be stretched across a hole near the corner of the board, and 

 tie them in that position with thread. Keep both upper and 

 under surfaces of the web well moistened with normal salt 

 solution, and cover a portion of the web, between two digits, 

 with a triangular piece of cover glass in order that it may 

 be examined with high as well as with low power. The 

 entire frog, excepting the foot, should be covered with a 

 wet cloth. Provide a box of equal height with the stage of 

 the microscope, and place the frog, stretched out on the 

 board, on the box and the stage. Adjust the board so that 

 the web of the frog's foot is in position to be viewed with 



