TttK ' METABOLIC BALANCE SHEET ' OV THE INDIVIDUAL TISSUES. 137 



The analysis may be uiidertakeu by one of three methods :— 



1. The blood gas-pump. 



2. The chemical method. 



3. The diti'erential method. 



1. The Blood Gas-pump. — No new principle has been introduced, but 

 extensive modifications have been gradually developed in technique in 

 devising what seems to be the most convenient form of piimp. 



It was essential to have apparatus which will work quickly and with 

 as little labour as possible. It has been found best to have four similar 

 pumps, which can be rendered vacuous a day or two before the experi- 

 ment. The blood is collected in long tubes of about 10 c.c. capacity. 

 The froth chamber is fitted with a double-surfac6 condenser filled with 

 ice and salt. The mercury is raised in the pump by water-pressure. 

 The 23umps are used as follow : (1) For the venous blood from the 

 resting organ ; (2) for the arterial blood from the resting organ ; (3) for 

 the venous from the active organ ; (4) for the arterial blood from the 

 active organ. 



2. The chemical method of analysing small samples of blood (1 c.c.) 

 lias been improved in a few manipulative details. 



3. Much of the analysis may be saved by adopting the difTerential 

 method, which gives a direct measurement of the difference between 

 the quantities of oxygen (or carbonic acid) in the arterial and venous 

 samples. This method, as well as being very rapid, is exceedingly accu- 

 rate, la general it will be found best to use the differential method, 

 and to test the accuracy of the results which it gives by the other two 

 methods. 



Ancesthetics. — The anaesthetic which is used requires careful considera- 

 tion. At present it will be best to avoid the use of volatile an;esthetics 

 (chloroform and ether) where the analysis is made by the blood gas- 

 pump. Morphia is often undesirable, both on account of its deleterious 

 effect on certain organs (the kidney) and on account of the deficient 

 respiration which it produces. In cases where none of the anaesthetics 

 mentioned can be used there remains urethane. 



Clotting of Blood.^A constant source of solicitude is the tendency of 

 the blood to clot in cannulse and apparatus. Two general methods for 

 the prevention of clotting seem useful. 



1. In cases where there is a rapid flow of blood from a big organ the 

 blood withdrawn may be received under oil into potassium oxalate. For 

 this purpose an exact method of dilution has been devised. 



2. In the case of small organs the animal may be given an injection 

 of hirudin. The Committee have ascertained that this reagent has no 

 specific disturbing influence on the gaseous contents of the blood. 



The Measurement of Rate of Flow has usually been made by the 

 direct observation of the time which the blood emerging from the organ 

 takes to fill a tube of convenient size. 



An indirect method which is valuable in certain cases, e.g., the 

 intestine, consists in placing the organ in a plethysmograph, damping 

 the vein momentarily, and noting the graphic record of the increase in 

 volume of the organ. 



Perfusion with Saline Solutions. — In some cases it is convenient to 

 measure the gaseous exchange of organs which are supplied with an 

 artificial circulation of Ringer's or Locke's solution. For this purpose 



