3 8 RESPIRA T/O.V 



Before making an observation, test whether the apparatus is air- 

 tight, as explained above, after introducing the animal into the cham- 

 ber, sealing the latter with wax, and connecting it with the absorption- 

 tubes. But a negative pressure of 2 or 3 inches of water is a sufficient 

 test for the small apparatus. 



To make an observation, set the air-current going at the desired 

 rate. Allow it to run for a few minutes till the carbon dioxide, which 

 has accumulated during the testing, has been swept out. At a time 

 which has been decided on and noted, stop the current by disconnecting 

 the water-pump. Disconnect and stopper up the animal chamber, and 

 weigh it as quickly as possible. Connect up again, using only recently- 

 weighed absorption-tubes, and finally connect with the water-pump 

 and allow the current to pass for a definite period, say an hour. 



The soda-lime should not be too dry, or absorption is not sufficiently 

 rapid. The following facts are made out: (a) Loss of weight by the 

 animal chamber (chiefly loss by the animal) ; (b) gain of the sulphuric 

 acid tube in water; (c) gain of the soda-lime tubes in carbon dioxide. 



The total loss and total gain do not correspond, the gain being always 

 greater than the loss. The surplus can only be oxygen absorbed by the 

 animal and added to the hydrogen and carbon of its substance to form 

 water and carbon dioxide. Calculate the respiratory quotient (p. 241). 



10. Muscular Contraction in the Absence of Free Oxygen (see p. 267). 

 Pith a frog (brain and cord). Cut off one hind-leg at the middle 

 of the thigh, and strip the skin from it. Pass a thread under the tendo 

 Achillis, tie it, and divide the tendon below it. Free the tendon and 

 the gastrocnemius muscle from the loose connective tissue lying between 

 them and the bones of the leg, and divide the latter just below the knee. 

 Remove superfluous thigh muscles, and fasten the gastrocnemius in 

 a moist chamber by means of the femur. Attach the thread on the 

 tendon to a lever. Connect the poles of the secondary coil of an induc- 

 tion machine by fine copper wires to the femur and the tendon. Put 

 a battery and simple key in the primary, and arrange it for single shocks. 

 Stimulate the muscle and observe the height of the contraction. Now 

 pass into the chamber a current of washed hydrogen gas from a bottle 

 containing granulated zinc, upon which a little dilute sulphuric acid 

 is poured from time to time. The air in the moist chamber will soon 

 be entirely displaced by the hydrogen, but the muscle will contract on 

 being stimulated, and the stimulation can be repeated many times. 



11. Oxidizing Ferments. Wash out the bloodvessels of a dog or 

 rabbit (Practical Exercises, p. 65). Chop up finely portions of pancreas, 

 spleen, muscle, lungs, and kidney, keeping each separate, and avoiding 

 any contamination of one by another. Grind up half of each portion 

 with sand in a small mortar, and extract with a small quantity of water, 

 keeping all the extracts separate. Into each of eleven test-tubes put 

 10 c.c. of a colourless dilute alkaline solution of paraphenylenediamin 

 and a-naphthol (freshly made by mixing solutions of the two sub- 

 stances in equimolecular proportions* and adding a little sodium 

 carbonate). To five of the tubes add the chopped organs, to five the 

 watery extracts of the organs, and enough water to make the volume 

 equal in all the tubes. To the remaining tube add the same amount 

 of water. Observe in which tube a change of colour takes place (p. 272). 



* I.e., the weight of each of the two substances in the mixture should be 

 proportional to its molecular weight. A convenient solution contains 0-144 

 per cent, of a-naphthol and o'io8 pei cent, of paraphenylenediamin. These 

 quantities are one-hundredth-molecular. Sod urn carbonate is added to the 

 amount of 0-25 per cent. The a-naphthol can be kept as a i per cent, solu- 

 tion in 50 per cent, alcohol. 



