146 PRACTICAL PHYSIOLOGY 



was determined by displacing the oxygen from laked ox blood 

 with ferricyanide of potassium, and measuring the amount of 

 gas. The percentage of haemoglobin corresponding to 18-5 per 

 cent, is about 13-8 per cent. The normal human blood when 

 saturated with CO and diluted with water to the mark 100 in tube 

 C corresponds in tint to the standard, and has therefore an oxygen 

 capacity of 18-5 per cent. 



Add distilled water to tube C up to the mark 20. Take exactly 

 20 c.mm. of blood in the pipette, and blow it into C. Pass a narrow 

 glass tube connected with a gas burner into the free part of tube 

 C. Turn the gas on and push the glass tube down near to the 

 blood. The gas tube is then withdrawn, and tube C quickly closed 

 with the finger to prevent the gas escaping. The tube is then 

 inclined up and down about a dozen times, so that the haemoglobin 

 becomes saturated with CO. 



Distilled water is then added drop by drop from the dropping 

 pipette A, until the tint appears equal to the standard. After 

 half a minute read the percentage, and then add another drop or 

 drops till the tints appear just unequal. Read the percentage 

 again, and take the mean of the two readings as correct. In com- 

 paring the tints hold the tubes against the skylight, and frequently 

 change the tubes from side to side. 



The Number of Corpuscles in the Blood. The Thoma-Zeiss 

 Haemacytometer consists of a counting chamber and an accurately 

 calibrated pipette. 



The finger behind the nail is cleaned with alcohol and ether, and 

 a drop of blood is drawn by the stab of a lancet-shaped needle. 

 The finger should not be constricted by a ligature during this 

 operation. The point of the pipette is placed in the drop, and the 

 blood is aspirated as far as the mark 1. The traces of blood on 

 the point of the pipette are then removed, and the pipette is dipped 

 into Hayem's fluid. 1 



This fluid is sucked up until the diluted blood reaches the mark 

 101. The tip of the mouth-piece is then closed by the finger, and 

 the pipette shaken. The glass bead in E mixes the blood and 

 Hayem's fluid. The bulb contains 1 part blood and 99 Hayem's 

 fluid. 



Now blow gently into the mouth-piece, reject the first few drops, 

 and then place a drop upon the centre of the counting chamber. 

 The cover-slip is then placed in position, and the counting chamber 

 is placed on the stage of the microscope, and left at rest for a few 

 minutes. When the corpuscles have subsided, count the number 

 in 10 squares, and take the average. Count those corpuscles which 

 happen to lie on the lines on two sides of each square only. Each 

 square covers an area of ^Q sq. mm., and has a volume of 40*00 

 c.mm., therefore 1 c.mm. contains 4000 times the average number 

 found in a square. The dilution of the blood was 1-100. Thus 



1 Sodium chloride, g. 2 ; sodium sulphate, g. 10 ; corrosive sublimate, 

 g. 1 ; water, g. 400, 



