a rabbit's veins, and felt almost sure of obtaining what

was required.

IMMUNITY-NATURAL AND ACQUIRED.

Too much stress is not to be laid upon inoculation experiments performed upon animals. The effects of bacteria are very variable. For instance, the bacillus of mouse septicæmia is fatal for house mice, whilst field mice are unhurt by it.1 Anthrax kills European sheep, but is resisted by Algerian, except in massive doses; 2 and Streptococcus pyogenes seems to be most pathogenic for mankind, although, like others, I have had difficulty in producing with cultures of it disease in rabbits and guinea-pigs. Anthrax is hardly at all pathogenic for dogs, although it kills sheep and cattle.1 But the natural immunity which some animals enjoy can be abolished by suitable means. For instance,

3

Pasteur found that fowls did not die of anthrax unless

1 Koch, "Traumatic Infective Diseases," New Sydenham Soc., translated by Watson Cheyne, 1880, p. 33.

2 Chauveau, Compt. rend. Acad. d. sc., Paris, 1879, tome lxxxix. p. 498.

66

3 Rosenbach, Micro-Organisms in Human Traumatic Infective Diseases," New Sydenham Soc., 1886, p. 408, translated by Watson Cheyne.

See papers by Pasteur, translated by Dawson Williams, "Microparasites in Disease," New Sydenham Soc., 1886. This is a most valuable volume, and contains several important monographs.

they were made ill, and their temperatures lowered by immersion in cold water. Frogs, on the other hand, were insusceptible to anthrax until they were kept in warm water. It is probable that similar influences affect the action of Staphylococcus aureus and of other bacteria.

Many of the experiments which are narrated require to be accepted with caution. Owing to their close resemblance, I have little doubt but that observers have hitherto failed to differentiate between some kinds of pathogenic and non-pathogenic cocci and bacilli. For instance, Bossowski1 has described a micrococcus which has the closest resemblance to Staphylococcus aureus, and which he calls Staphylococcus gilvus. It liquefied gelatine more slowly than aureus, and was non-pathogenic for rabbits. It is said that the wounds in which it was found did not suppurate.

Under the name of Staphylococcus albus many kinds of bacteria have doubtless been included. Welch 2 has described a micrococcus which is a constant inhabitant of the skin, and which he calls Staphylococcus epidermidis albus. It bears the same relation to Staphylococcus albus as gilvus does to aureus.

1 Loc. cit., p. 256.

2 "Conditions underlying the Infection of Wounds," Am. Journ. Med. Sc., Phila., November 1891.

Independently of Welch, I have myself1 described a Diplococcus epidermidis albus which is very like Staphylococcus pyogenes albus, and was found in the vicinity of antiseptic wounds. I have very frequently met with it since, both in wounds and in broth into which skin has been dropped. I am by no means sure that it is the same as the Staphylococcus epidermidis. It produces a peculiar odour as it grows, such as is smelt when uncleanly people remove their vestments. It seems to have no effect upon rabbits when injected into their blood, nor does it cause suppuration when squirted under the skin.

Staphylococcus pyogenes albus, which has been referred to so often, has the closest resemblance to Staphylococcus aureus in everything except colour. Grown upon culture media, however, it lacks the golden hue, and is white, as its name implies. Much remains to be done to clearly differentiate Staphylococcus albus from the various bacteria which resemble it so closely in everything but its pathogenic properties.

1 "Further Report on Aseptic and Septic Surgical Cases, with special reference to Infection from the Skin," Brit. Med. Journ., London, 28th May 1892.

CHAPTER IV.

MICROCOCCI OF WOUNDS.- -Continued.

STREPTOCOCCUS PYOGENES.

Staphylococcus pyogenes aureus having been described, we may now proceed with an organism equally important, the Streptococcus pyogenes. At the same time, the methods of staining which dressers or clerks ought to be able to practise without difficulty will be mentioned, also how to make cover-glass preparations. These are so simple and satisfactory, that it is to be hoped that everyone may be incited to attempt them in future more often than at present.

The name Streptococcus was given by Ogston to this species of coccus, because of the chains in which it grows. In suitable media, such as sterilised broth, the chains attain their full dimensions, and in those of young and vigorous cultures I have counted upwards of two hundred cocci linked together in a tortuous chain. More commonly a chain contains from five to

ten elements.

The average size of the cocci is about 1 μ, but in the midst of the long chains some may be seen which are twice as big, and which are called arthrospores. In the centre of some of the arthrospores I have seen small, round, shining bodies which appeared to be spores. I have never seen arthrospores in the streptococci in pus or in the tissues; only in those growing in culture media. When we come to

disinfection, spores will be of great importance.

To obtain a pure culture of Streptococcus pyogenes, it is often sufficient to inoculate the pus of an acute inflammatory process into gelatine or broth. A case

of cellulitis is the most favourable, because, as Ogston long ago observed, the streptococcus is peculiarly associated with that condition. The finest chains which I have grown in broth came from a pelvic cellulitis. The growth on gelatine is very typical; no liquefaction occurs. An almost imperceptible growth advances over the surface. It consists of minute greyish white and almost transparent colonies, with abrupt and undulating edges. These colonies have hardly any tendency to spread; their growth soon ceases, and they remain about 1 mm. in diameter. Inoculated by puncture into the depths of the gelatine, similar colonies appear, free oxygen not being essential for the growth of Streptococcus pyogenes. In broth the