[Illustration:  FIG. 36.—­Spirochaeta pallida from scraping of hard Chancre of Prepuce. x 1000 diam.  Burri method.]

The spirochaete may be recognised in films made by scraping the deeper parts of the primary lesion, from papules on the skin, or from blisters artificially raised on lesions of the skin or on the immediately adjacent portion of healthy skin.  It is readily found in the mucous patches and condylomata of the secondary period.  It is best stained by Giemsa’s method, and its recognition is greatly aided by the use of the ultra-microscope.

The spirochaete has been demonstrated in every form of syphilitic lesion, and has been isolated from the blood—­with difficulty—­and from lymph withdrawn by a hollow needle from enlarged lymph glands.  The saliva of persons suffering from syphilitic lesions of the mouth also contains the organism.

[Illustration:  FIG. 37.—­Spirochaeta refrigerans from scraping of Vagina. x 1000 diam.  Burri method.]

In tertiary lesions there is greater difficulty in demonstrating the spirochaete, but small numbers have been found in the peripheral parts of gummata and in the thickened patches in syphilitic disease of the aorta.  Noguchi and Moore have discovered the spirochaete in the brain in a number of cases of general paralysis of the insane.  The spirochaete may persist in the body for a long time after infection; its presence has been demonstrated as long as sixteen years after the original acquisition of the disease.

In inherited syphilis the spirochaete is present in enormous numbers throughout all the organs and fluids of the body.

Considerable interest attaches to the observations of Metchnikoff, Roux, and Neisser, who have succeeded in conveying syphilis to the chimpanzee and other members of the ape tribe, obtaining primary and secondary lesions similar to those observed in man, and also containing the spirochaete.  In animals the disease has been transmitted by material from all kinds of syphilitic lesions, including even the blood in the secondary and tertiary stages of the disease.  The primary lesion is in the form of an indurated papule, in every respect resembling the corresponding lesion in man, and associated with enlargement and induration of the lymph glands.  The primary lesion usually appears about thirty days after inoculation, to be followed, in about half the cases, by secondary manifestations, which are usually of a mild character; in no instance has any tertiary lesion been observed.  The severity of the affection amongst apes would appear to be in proportion to the nearness of the relationship of the animal to the human subject.  The eye of the rabbit is also susceptible to inoculation from syphilitic lesions; the material in a finely divided state is introduced into the anterior chamber of the eye.

Attempts to immunise against the disease have so far proved negative, but Metchnikoff has shown that the inunction of the part inoculated with an ointment containing 33 per cent. of calomel, within one hour of infection, suffices to neutralise the virus in man, and up to eighteen hours in monkeys.  He recommends the adoption of this procedure in the prophylaxis of syphilis.