Distinguish the pulmonary arteries from the pulmonary veins. Probe into the 

 cut vessels leading into and out of the heart. Through which of these can you 

 push a pencil.'' Compare the thickness of the walls of the veins and of the arteries. 

 Lay open the side of the aorta by cutting with scissors. Note the structure of the 

 semilunar valves. 



Cut the heart open so as to expose the four valves. Compare the thickness of 

 the auricle walls and ventricle walls. Trace the passage of the blood, as it moves 

 through the heart, past various openings. 



3 To observe the flow of blood in living tissues, watch the web of a frog's 

 foot through a microscope, first under the 16-millimeter objective and then under 

 the 4-millimeter objective. Note that the blood moves rapidly in some vessels, 

 slowly in others, and that the pulsation can be seen in some but not in others. 

 Find places where arterioles branch to form capillaries, and places where capillaries 

 are joined into small veins. Observe the extent to which capillaries reach all parts 

 of the tissue. 



4 To demonstrate the "buffering" action of various compounds, treat solu- 

 tions of "buffer salts" with measured quantities of acid and of alkali, and compare 

 with the action of plain water. 



Use (a) plain water as control, or basis of comparison, and make up four solu- 

 tions as follows: In 200 cc of water dissolve (b) 1 teaspoonful of baking-soda 

 (NaHCOs) ; (c) 1 teaspoonful of dibasic sodium phosphate (Na^-HPOi) ; (d) 1 

 teaspoonful of monobasic sodium phosphate (NaHoP04) ; (e) \ teaspoonful each 

 of dibasic sodium phosphate and monobasic sodium phosphate. 



As an indicator use extract of red cabbage. {Boil the leaves in water to extract 

 the red juice.) When acid, this extract has a pink color; when neutral a blue color; 

 and when basic, a green color. 



Prepare five sets of three containers each, using 100-cubic-centimeter beakers or 

 small tumblers or bottles (all of the same diameter, to make comparisons of colors 

 easier). Place 50 cc of water in each beaker of set a; 50 cc of baking-soda solution 

 in each of set i^; 50 cc of dibasic sodium phosphate solution in each of set c; 50 cc 

 of monobasic sodium phosphate in each of set d', and 50 cc of the mixed dibasic 

 and monobasic sodium phosphate in each of set e. Add 10 cc of the cabbage 

 extract to each vessel. 



Compare the colors of five sets of the solutions. Note that some are slightly 

 alkaline, some are neutral, and some slightly acid. Record the state of each. Set 

 up one burette with a half-and-half mixture of hydrochloric acid and water, and a 

 second beaker with a half-and-half mixture of concentrated ammonia and water. 

 Add acid, a drop at a time, to one of the beakers having water (fli) until there is 

 a pink color (two drops should be enough). Add sufficient base to the second 

 water beaker {a-,) to give a barely green color (two drops ought to be enough). 

 Add enough drops of acid to one of the vessels in each of the four other solutions 

 {bi, T], ^1, e^) to give the same pink color shown by the acid water solution (ai) 

 and record the amount of acid each required. Record the number of drops of base 

 required by each of the four solutions bo, Co, do, and €■>, barely to give the green 

 color of the basic water solution (a^). Compare the number of drops of acid and 



199 



