236 INFECTIOUS AND PARASITIC DISEASES. 



From an examination of the blood alone, 

 Blood the diagnosis of a limited number of dis- 



Specimens. eases can often be made. In a few, as 

 for example malaria, the agents of the dis- 

 ease can be detected with the microscope in properly 

 prepared specimens; in others, e.g., typhoid fever, 

 when a drop of blood is brought in contact with known 

 typhoid bacilli, a characteristic phenomenon, WidaVs 

 reaction^ takes place, which consists in a clumping or 

 drawing together of the bacilli; in still another class of 

 diseases, the diagnosis is made by determining the 

 number of red and white corpuscles present in one 

 cubic centimeter of blood, and noting in what respects 

 their number and appearance differ from that which is 

 normal. In cities maintaining municipal labora- 

 tories, outfits for the collection of specimens of blood 

 for the diagnosis of both malaria and typhoid fever are 

 supplied. 



This outfit for typhoid fever consists of 

 Typhoid either a slip of unglazed paper, a glass 

 Fever slide, or a metal plate, upon which the 

 (Widal) blood is to be collected, and a needle for 

 Outfit. puncturing the ear or finger. These with 

 the directions for taking the blood are 

 enclosed in an envelope. 



The directions for taking the blood-drop on paper 

 are as follows: 



^'Thoroughly cleanse the skin of the patient's finger- 



