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an atmosphere of nitrogen (obtained by absorbing oxygen by alkaline permanganates), hydrogen, or carbon dioxide gas. The growth takes place in ordinary media, but the addition of grape sugar gives better results.
In glucose-gelatine stab cultures the growth takes place along the track of the needle, but at a considerable distance below the surface, in the form of a radiating outgrowth. The medium is liquefied, and some gas may be produced. In gelatine plates the colonies are very characteristic : there is an opaque central portion which is surrounded by a series of radiating filaments. The growth also occurs in bouillon and gives off a peculiar fætid odour.
The spores of the tetanus bacillus are extremely resistant. They withstand desiccation for months, and are uninjured after exposure to 80° C. for one hour, but are rapidly killed at 100° C. They resist for many hours the action of 5 per cent. carbolic acid, or 1 in 1,000 of corrosive sublimate; but the addition of a little hydrochloric acid to these substances readily brings about their destruction.
The bacillus leads a saprophytic existence in garden soil and in dung heaps, whence it finds its way into the human organism. But the bacillus alone is unable to give rise to infection—it must be introduced with the pyogenic cocci (mixed infection), or there must be some injury to the tissues before it is enabled to gain a foothold and produce disease.
The organism is not found in the blood and tissues, but remains localised at the seat of inoculation. In this situation it manufactures the toxins which enter into direct combination with the central nervous system, and thus give rise to the characteristic symptoms of the disease. Wassermann, by mixing tetanus toxin with an emulsion of spinal cord, found, on inoculation into guinea
pigs, that the mixture was no longer toxic. This shows that there exist in the central nervous system certain molecules with a combining affinity for the tetanus toxin. The experiment affords an interesting confirmation of Ehrlich's hypothesis of immunity (see p. 124).
The results of serum treatment are not so satisfactory as in diphtheria. This is due to the fact that, unlike the latter disease, tetanus cannot be diagnosed sufficiently early to allow the antitoxin to have its full scope. An apparently trivial wound is often neglected, and the patient does not come for treatment until the disease is far advanced.
Attempts have recently been made to obtain better results by the intracerebral injection of antitoxin, which is also injected hypodermically at the same time.
“ The intracerebral injection immunises the higher nerve centres before the toxin has been fixed there. The antitoxin given hypodermically renders the blood antitoxic, and the toxin, as it becomes absorbed from the source of supply-wound, bruise, or any other source—is neutralised as soon as it enters the blood ” (Semple).
A few successful cases have been recorded as the result of this treatment.
The parasite which is now universally acknowledged as the cause of malarial fevers is not a bacterium, but a protozoon called plasmodium or hemamaba malarie. This organism, which is quite unlike any we have considered before, was discovered by Laveran in 1880. He noticed that it developed in, and at the expense of, the red-blood corpuscle, which was finally reduced to a mere shell, the parasite appropriating the hæmoglobin and blooming into a "rosette". Subsequently, Golgi discovered that these marguerite-like bodies represent the reproductive stage of the parasites, which, moreover, were of various kinds. He also demonstrated the remarkable fact that the malarial paroxysm always coincided with the sporulation of a group of parasites. This cleared up the mystery of the periodicity of this disease; but an important question still remained to be answered. How did these organisms enter the human body ? Manson, in 1896, suggested that the mosquito probably subserved the malarial parasite in the same way as it did in the case of filaria nocturna. The truth of this hypothesis was subsequently confirmed by Ross's experiments, which showed that a particular species of mosquito served as the carrier of this disease.
The malarial parasite is a unicellular organism consisting of protoplasm, a nucleus, and a nucleolus. It possesses
Fig. 24.–Schema showing the Human and Mosquito Cycles of the
two cycles of development, one in man and the other in the mosquito. In the human host it propagates by the asexual mode of reproduction ; but the species is perpetuated by its passage through the mosquito, in which the reproduction is a sexual one. As has just been stated, it is by means of this insect that the parasite passes from man to man.
Developmental Cycle in Man.-In its earliest stages the malarial organism is seen as a hyaline ameboid body in the substance of the red corpuscle. The organism increases in size and converts the hæmoglobin into melanin, which it appropriates to itself.
When the full maturity is obtained it becomes either a sporocyte (sporulation form), or a gametocyte (sexual form). In the former case it divides into a number of segments or spores, which, by the rupture of the corpuscular host, are set free to attack fresh corpuscles, and there to undergo a similar cycle of changes. The melanin, which is also discharged into the blood plasma along with the spores, is ingested by the leucocytes. The sporulation of a group of parasites is accompanied by a paroxysm, which is probably due to certain toxins liberated at the same time.
The Mosquito Phase. The gametocytes, on the other hand, show no segmentation, but circulate unchanged in the blood. They represent the sexual forms of the organisms, and become active when transferred to their definitive host, the mosquito. When, therefore, they are imbibed by a mosquito biting a malarial subject, they burst from their corpuscular host, and lie free in the stomach cavity of the mosquito. The male gametocyte now emits a number of active motile filaments called microgametes. Some of these spermatozoon-like bodies become detached, and entering the female sexual elements, bring about their fecundation. The resultant fertilised body becomes