Tetanus has been suspected to be an infective disease by many (see former editions of this work). The demonstration and full elucidation of its nature have been effected of late years. The tetanus bacillus is a slender rod. Under certain circumstances it produces spores, and these are peculiarly situated at the ends of the rods so that there is the appearance of a drum-stick. The spores are very resistant, even to considerable heat. The bacillus is readily stained by the ordinary dyes and also by Gram's method.

The bacillus or spores are present in earth, dust, and sometimes in the faeces of animals. It grows in the ordinary culture media, but is strictly anaerobic. The cultures present fine brush-like prolongations. They emit a peculiar odour. In the cultures the virulent toxine is produced, and the symptoms of tetanus are producible by injection of it free from bacilli. The nature of the toxine is not definitely known. It is destroyed by heat.

The microbe grows locally but produces little local disturbance, at most some hyperemia but no suppuration. It does not propagate in the blood. The symptoms are produced by the absorption of the toxine into the blood. It is highly pathogenic in the horse and guinea-pig, and in diminishing ratio in the mouse, rabbit, rat, etc. Fowls seem to be immune except to very large doses.

An interesting peculiarity is that pure cultures when washed so as to free them from the toxine are not infective. In order to its growth in the tissues the microbe requires either some foreign matter which will act on the tissue, such as lactic acid, or the company of some other microbe'which will assist it in combating the tissues. In actual cases in man the wound which leads to tetanus contains foreign matter and microbes, and usually suppurates, but not from the action of the tetanus bacillus.

Immunity is produced artificially in animals and the serum of immunized animals contains an antitoxine which protects other animals from the action of the toxine.