THE BLOOD 121 



of guaiacum and hydrogen peroxide, the blood causing the red tincture 

 to become green, is not very trustworthy, as it is also given by many 

 other organic substances ; it is, however, with certain precautions, 

 used by some. 



In medico-legal cases it is often necessary to ascertain whether 

 a red fluid or stain upon clothing is or is not blood. In any such 

 case it is advisable not to rely upon one test only, but to try every 

 means of detection at one's disposal. To discover whether it is blood 

 or not, is by no means a difficult problem, but to distinguish human 

 blood from that of the common mammals is possible only by the 

 ' biological ' test described at the end of the next section. 



IMMUNITY 



The chemical defences of the body against injury and disease 

 are numerous. The property that the blood possesses of coagu- 

 lating is a defence against haemorrhage ; the acid of the gastric 

 juice is a protection against harmful bacteria introduced with food. 

 Bacterial activity in urine is inhibited by the acidity of that secretion. 



Far more important and widespread in its effects than any of the 

 foregoing is the bactericidal (i.e. bacteria-killing) action of the blood 

 and lymph ; a study of this question has led to many interesting 

 results, especially in connection with the important problem of 

 immunity. 



It is a familiar fact that one attack of many of the infective maladies 

 protects us against another attack of the same disease. The person 

 is said to be immune, either partially or completely, against that 

 disease. Vaccination produces in a patient an attack of cowpox or 

 vaccinia. This disease is either closely related to smallpox, or 

 may be it is smallpox modified and rendered less malignant by passing 

 through the body of a calf. At any rate, an attack of vaccinia renders 

 a person immune to smallpox for a certain number of years. Vac- 

 cination is an instance of what is called protective inoculation, which 

 is now practised with more or less success in reference to other 

 diseases, such as plague and typhoid fever. The study of immunity 

 has also rendered possible what may be called curative inoculation, or 

 the injection of antitoxic material as a cure for diphtheria, tetanus, 

 snake poisoning, &c. 



The power the blood possesses of slaying bacteria is not limited 

 to the colourless corpuscles OY phagocytes, but is also a property of the 

 fluid part of the blood, at any rate in the case of some micro- 

 organisms. The chemical characters of the substances which kill 

 the bacteria are not fully known ; but they appear to be protein 



