Of late years it has been found that these antitoxins can be artificially supplied or caused to develop. Thus the system may be aided in neutralizing the effect of the toxin, and in warding off the disease. By injecting these antitoxins or stimulating their development, people are now protected against smallpox, diphtheria and other diseases.
Disease bacteria are not only found in the air, but also in water and milk and other kinds of food. We must therefore be very careful to keep these germs from our water, milk and food supply. Many cases of typhoid fever have been directly traced to the milk supplied by a dealer in whose family was a case of the fever. Flies (Fig.
Fig. 106.
106) are great carriers of bacteria and, by
crawling over food, spread diseases.
Germs thrive particularly in sewers, cesspools and unsanitary places, so that these should be especially watched. The best guards against disease, however, are plenty of sunshine and air, wholesome food, sufficient rest and a tranquil mind. With these aids, the body is usually prepared in itself to kill the germs that come into it. Every day each person probably receives into his system thousands of disease germs and usually it is only when the vitality of the body is low that these germs are able to establish themselves. Right living is the great disease preventer.
As has already been stated, however, disease bacteria are only a small portion of the bacteria group of plants and the usefulness of theother members of this group is far greater than their harmfulness. Science each year is becoming more and more able to fight the disease germs, but it is entirely unable to supply the necessary aid given by the useful bacteria to animals and plants and, through them, to man.
100.Animals. -Animals do not take their energy directly from thesunlight, but indirectly from the latent energy stored up in the foods prepared by green plants. These foods may be eaten as stored by the plants, or they may have passed through the medium of other plants and animals. The energy thus stored up is liberated by combining the carbon with oxygen. Carbon dioxide is freed.
The green plants use this carbon dioxide again and, by the aid of the sun"s energy, free the oxygen and store up the carbon. Thus the cycle goes on, over and over, the plants freeing oxygen and taking up carbon dioxide, and the animals freeing carbon dioxide and taking up oxygen. The cells of plants which feed upon the food prepared by the chlorophyll of the leaves use oxygen and give out carbon dioxide just as the animal cells do; so also do other plants to some extent, but this is in small quantities.
101.Classification of Animals. -For convenience of study theanimal kingdom has been divided into two great classes-the inver- tebrates (without backbone) and the vertebrates (with backbone). The invertebrate is the much more numerous class as it contains the worms, shellfish, insects and those almost countless forms of animal life which have no internal bony skeleton and backbone. The higher animals, like fishes, amphibia, reptiles, birds and mammals, belong to the class of vertebrates. Man himself is the highest of the vertebrates, and as the purpose of this book is to study the earth and its relation to man, his structure will be studied later.
102.Invertebrates; Protozoa. -The very lowest form of animal life, the protozoa, are single-celled animals. In some species they are very difficult to distinguish from plants. They are microscopic in size and most of them live in water. Our chief interest in them in the pres- ent study is that they are the cause of several kinds of disease whichcan readily be prevented with proper care. Malaria, and the terrible African disease called the sleeping sickness, and probably yellow fever are due to these little animals.
A DISEASE-BEARING MOSQUITO.
Greatly magnified.
Unlike bacteria, the protozoa do not cause disease by passing directly from one person to another, instead they need to live in some insect between whiles. In malaria and yellow fever the insect in which they live is the mosquito, and in the sleeping sickness they live in a fly calledthe tsetse. If a mosquito of the right species bites a person afflictedwith malaria or yellow fever, some of these little animals, the protozoa, are sucked up with the blood and enter the mosquito. They grow in its body, undergoing several changes, until the animal germs are ready to be injected into their victim, when they pass into the salivary glands of the mosquito. In biting, the mosquito always injects a little saliva into the wound and with this go the germs. These enter the blood, multiply rapidly and cause the disease.
If mosquitoes can be kept from biting people who have these diseases or if infected mosquitoes can be kept from biting other people, such diseases will not spread. The best way to keep mosquitoes from biting is to exterminate them. Sincemosquitoes breed in stagnant water, all old dishes or small pools where water accumulates should be emptied and drained. Larger stagnant pools should be drained or have a film of kerosene spread over theirsurface by frequently pouring a
little of the oil on the water. This
A " MALARIAL " SWAMP.
A breeding place for mosquitoes.
will keep the mosquitoes from breeding and prevent the diseases.
Thus mosquitoes and flies, the summer pests, are not only exceedingly annoying, but are very likely to spread disease. The Texas fever which has caused such great financial losses to the cattlemen of the United States is caused by a protozoan injected into the cattle by the bite of a tick.
103.Worms. -Another class of invertebrates is the worms. One ofthese, the earthworm, was found in the study of soilmaking to be very important and should be considered in this place. If an earthworm isEARTHWORM.
A great helper of the farmer.
