Strychnin is selective in its action, in that it stimulates the respiratory centers and the motor nerves. Bromids depress them.

Foreign bodies that are not living may cause disease by mechanically interfering with the functions of the body. The most important causes of disease are, however, bacteria, the lowest forms of vegetable life. They are almost ubiquitous and give rise to many disturbances of function. It is not, however, always possible to prove the relationship between bacteria and disease. Koch has advanced four laws. They are: (I) The bacteria must be found in the diseased individual; (2) they must be capable of cultivation upon media outside the body; (3) pure cultures introduced into a healthy animal must produce the disease in the animal; (4) the bacteria must be recovered from the inoculated animal.

Diseases caused by bacteria are capable of transmission from person to person and are generally termed infectious. They may gain entrance into the body through abrasions of the skin and mucous membranes, through the air or by means of the digestive tract, through the genito-urinary tract, or they may be transferred from the maternal to the fetal blood in the uterus.

Animal parasites may frequently be the cause of disease. To this class belong the various intestinal worms and certain blood organisms, as the plasmodium of malaria, the filaria, the trypanosomes, etc.