200 INTRODUCTION TO GENERAL PHYSIOLOGY 



the frog lying on its lower side. Cutting carefully through the 

 tissues under the skin with fine pointed scissors, the nerve will be 

 found. Pass a thread under all the nerve trunks seen and, raising 

 them by the thread, not yet tied, follow up with the scissors to 

 the place where they leave the spinal column. Tie here and cut 



between the ligature and the bone. Dur- 

 ing the experiment keep the skin moist 

 C f^s\ with 0.7 per cent, sodium chloride. 



Respiration 



Trachea of Insect. Any small piece of 

 tissue cut from the interior of an insect 

 and spread out on a microscope slide in 

 0-7 per cent, saline will show the branch- 

 ing system of tubes containing air. 



Haemoglobin. The carriage of oxygen 

 by the red corpuscles of the blood can be 

 shown by a simplified vacuum pump 

 made by a glass-blower. Fig. 19 shows 

 the pattern, which will be found useful for 

 many purposes. 



With the stopcock K open to the cup 

 C, and D making communication between 

 A and E, raise the mercury reservoir F 

 until a little mercury has entered C. Close 

 B and lower the reservoir until the mercury 

 leaves E, that is 760 mm. below E. There 

 is now a Torricellian vacuum in A and E. 



Take about 10 to 15 c.c. of blood, 

 which should be fresh, and either defibrin- 

 ated by stirring with a feather, or pre- 

 vented from clotting by the addition of a 

 small amount of powdered potassium oxa- 

 late. Sufficient will be otained from a rat 

 killed by cutting its throat. If defibrinated, 

 it will need straining through muslin to 

 remove bits of fibrin. 

 Allow a known volume of this blood to run into the vacuum by 

 putting it in the cup C and turning the stopcock slowly. It will be 

 seen to froth and to become more crimson in colour. Raise the 

 reservoir, after closing the stopcock, until the blood, neglecting the 

 froth, just fills the vessel E. Then turn the stopcock D so that the 

 blood is driven out. Collect it in a small bottle. Bring the mercury 

 reservoir into connection with A again, and in such a position that 



FIG. IQ. Vacuum Pump 

 for Blood Gas Experi- 

 ments. 



A, graduated tube. 

 13, stopcock. 



C, cup. 



D, three-way stopcock. 



E, reservoir. 



F, mercury vessel. 



