174 UNCONSCIOUS NERVOUS OPERATIONS 



made that will keep permanently in suitable preservative fluids. But 

 the larger blood vessels can be easily dissected out in iminjected 

 specimens. 



77. Structure of Blood Vessels. Prepared cross sections of the walls 

 of blood vessels can be purchased, or borrowed from some local 

 physician. The blood vessel is seen to be composed of three coats : 

 inner, epithelial; middle, largely muscular; and outer, fibrous. 



78. Circulation in the Web of the Frog's Foot. The frog should be 

 placed in a small cloth bag, one foot being allowed to protrude. The 

 animal should then be tied upon a board or wooden frame with the 

 web of its foot stretched over an opening in the board. The web 

 may be kept stretched by the aid of strings tied to the toes. The 

 apparatus should then be placed so that the web over the hole in the 

 frame lies directly under the objective of a compound microscope. If 

 the animal, and especially the web of the foot, be kept moist, and the 

 cords confining it are not too tight, the movements of the blood 

 corpuscles can be studied for a long time. The experiment may be 

 performed with little discomfort to the frog. 



79. Valves in Veins can be shown by pressing firmly with the finger 

 upon one of the veins of the forearm, and then passing the finger 

 up along the vein toward the hand. The positions of the valves are 

 indicated by the temporary swellings that make their appearance as 

 the blood is forced back against the flaps of the valves. 



80. Scheme of the Circulation. The general features of the circu- 

 lation can be illustrated with the apparatus shown in Fig. 98. The 

 mechanism can be easily constructed, requiring only some rubber and 

 some glass tubing, a few glass Y-tubes, some pinchcocks, and a bulb 

 syringe. 



81. To illustrate Arterial and Venous Pressure and Flow. Remove 

 the clamp from tube C, and force water through the apparatus with 

 quick, regular strokes. The mercury in the manometers rises and falls 

 with each stroke, and the water issues in jets from E. Clamp C. and 

 continue as before. The mercury in manometer M oscillates out 

 rises higher than before, with a marked excess of rise over fall, so 

 that finally the mercury in one limb of the manometer stands at a 

 considerable height, showing vibrations with each stroke of the pump. 

 In manometer N the mercury rises slowly, with little or no oscillation, 

 but the pressure is not so great as in M. The water issues from E 

 in a steady stream. Open clamp at Z>; the water issues in jets corre- 



