BLOOD SEKUM 63 



(14) Potato-tubes. — Large and sound potatoes are tho- 

 roughly scrubbed until clean, and then with a large cork- 

 borer cylindrical pieces are bored to fit into test-tubes. 

 Each cylinder is cut into two halves diagonally, and each 

 half placed in a test-tube. It is advisable to allow the 

 cores of potato to rest on a moist plug of cotton-wool ; this 

 will keep the potato-cylinder moist. The tubes are then 

 plugged and capped as usual, and sterilised for thirty 

 minutes in the steam steriliser on three successive days. 



Potatoes form an excellent culture medium for many 

 organisms, and one which secures their development in a 

 very characteristic way. Care should be taken to prevent 

 overheating, otherwise the potatoes will lose their natural 

 white colour, and become sodden in appearance. Potatoes 

 possess a slightly acid reaction, so the surface is rendered 

 faintly alkaline with sodium bicarbonate for the growth of 

 certain micro-organisms whose growth requires alkalinity. 



(15) Blood Serum. — Blood serum is an excellent medium 

 for all the pathogenic organisms, all of which grow with 

 greater rapidity on this medium than on any other. It 

 does not matter much what animal the blood is taken from, 

 but the blood of the horse yields the lightest-coloured 

 serum. Blood from horses, bullocks, pigs, and sheep is 

 generally to be obtained from slaughter-men. The blood 

 from the jugular vein or an incised wound is allowed to 

 run into a tall glass vessel. The vessel containing the 

 blood is at once placed in a cool place without the least 

 shaking, and allowed to stand overnight, when it will be 

 found that a firm clot has formed ; the clear serum is then 

 drawn off by means of a glass siphon or large pipette. The 

 serum is rendered faintly alkaline and run off into test- 

 tubes, which are then plugged as usual, and laid on a 

 slanting surface, and the serum made to set by heating in 

 the hot-air steriliser to 75° C. The tubes can then be 



