78 METHODS OF CULTIVATION OF BACTERIA. 



medium is then taken, and by a simple calculation the 

 total number of colonies present can be obtained. Plate- 

 cultures in Petri's dishes are sometimes employed for pur- 

 poses of counting. The bottoms of such dishes are, how- 

 ever, never flat, and the thickness of the medium thus 

 varies in different parts. If these dishes are to be used, 

 a circle of the same size as the dish can be drawn with 

 Chinese white on a black card, the circumference divided 

 into equal arcs, and radii drawn. The dish is then laid 

 on the card, the number of colonies in a few of the sectors 

 counted, and an average struck as before. In counting 

 colonies it is always best to aid the eye with a small hand 

 lens. 



The Bacteriological Examination of the Blood. This is 

 usually done by taking a small drop from the skin surface, 

 e.g., the lobe of the ear. The part should be thoroughly 

 washed with i-iooo corrosive sublimate and dried with 

 sterile cotton wool. It is then washed with absolute 

 alcohol to remove the antiseptic, drying being allowed to 

 take place by evaporation. A prick is then made with a 

 sterile surgical needle, the drop of blood is caught with a 

 sterile platinum loop and smeared on the surface of agar 

 or blood serum. It is rare to obtain growths from the 

 blood of the human subject (vide special chapters), and if 

 colonies appear the procedure should be repeated to exclude 

 the possibility of accidental contamination. When larger 

 quantities of blood are obtainable, e.g., by venesection, 

 suitable animals may be inoculated with several cubic 

 centimetres. In this way infection may be produced from 

 even human blood, e.g., when the subject is suffering from 

 the effects of the pathogenic streptococci. 



In examining the blood of the spleen a portion of the 

 skin over the organ is sterilised in the same way, a few 

 drops are withdrawn from the organ by a sterile hypodermic 

 syringe and cultures made. (For microscopic methods, 

 vide p. 96.) 



The Bacteriological Examination of Urine. In such 

 an examination care must be taken to prevent the con- 



