It is a small rod-shaped organism with short flagellae, which render it motile (Fig. 4).  It closely resembles the typhoid bacillus, but is distinguished from it by its behaviour in artificial culture media.

[Illustration:  FIG. 5.—­Fraenkel’s Pneumococci in Pus from Empyema following Pneumonia. x 100 diam.  Stained with Muir’s capsule stain.]

Pneumo-bacteria.—­Two forms of organism associated with pneumonia—­Fraenkel’s pneumococcus (one of the diplococci) (Fig. 5) and Friedlander’s pneumo-bacillus (a short rod-shaped form)—­are frequently met with in inflammations of the serous and synovial membranes, in suppuration in the liver, and in various other inflammatory and suppurative conditions.

Bacillus Typhosus.—­This organism has been found in pure culture in suppurative conditions of bone, of cellular tissue, and of internal organs, especially during convalescence from typhoid fever.  Like the staphylococcus, it is capable of lying latent in the tissues for long periods.

Other Pyogenic Bacteria.—­It is not necessary to do more than name some of the other organisms that are known to be pyogenic, such as the bacillus pyocyaneus, which is found in green and blue pus, the micrococcus tetragenus, the gonococcus, actinomyces, the glanders bacillus, and the tubercle bacillus.  Most of these will receive further mention in connection with the diseases to which they give rise.

#Leucocytosis.#—­Most bacterial diseases, as well as certain other pathological conditions, are associated with an increase in the number of leucocytes in the blood throughout the circulatory system.  This condition of the blood, which is known as leucocytosis, is believed to be due to an excessive output and rapid formation of leucocytes by the bone marrow, and it probably has as its object the arrest and destruction of the invading organisms or toxins.  To increase the resisting power of the system to pathogenic organisms, an artificial leucocytosis may be induced by subcutaneous injection of a solution of nucleinate of soda (16 minims of a 5 per cent. solution).

The normal number of leucocytes per cubic millimetre varies in different individuals, and in the same individual under different conditions, from 5000 to 10,000:  7500 is a normal average, and anything above 12,000 is considered abnormal.  When leucocytosis is present, the number may range from 12,000 to 30,000 or even higher; 40,000 is looked upon as a high degree of leucocytosis.  According to Ehrlich, the following may be taken as the standard proportion of the various forms of leucocytes in normal blood:  polynuclear neutrophile leucocytes, 70 to 72 per cent.; lymphocytes, 22 to 25 per cent.; eosinophile cells, 2 to 4 per cent.; large mononuclear and transitional leucocytes, 2 to 4 per cent.; mast-cells, 0.5 to 2 per cent.