The science of life : fully illustrated in tone and line and including many diagrams

THE GOMPLEX BODY-MACHINE AND HOW IT WORKS

throat, and the appendix at the entrance to the large intestine also, are composed entirely of leucocyte-forming tissue of this kind. They are particularly strong salients on the line of defensive fortification. So at nearly every vulnerable point Mr. Everyman is garrisoned against his smallest (and greatest) enemies.

It is not only against positive infection that his multitudinous commonweal must defend itself. Mr. Everyman is everlastingly resisting certain physical processes, as persistent and insidious as his bacterial foes. First, like the mouse and all other mammals and birds, he has to keep himself warm inside. He belongs to a large and dominant sort of machines called warm-blooded creatures, of a distinctive delicacy and versatility. It is only within a very limited range of temperature that his protoplasm can work at its best. That is probably true of all living things, but while the warm-blooded creatures keep going as a rule in almost all the weather changes to which they are subjected, the cold-blooded creatures lie up and become inactive when it is too cold or too hot. Their bodies acquiesce in the temperature of their surroundings and all that it implies in sluggishness or liveliness, while the warmblooded creatures are generally warmer than their surroundings and will not consent so readily to follow the intimations of the thermometer. The temperature of a healthy man’s blood is about 37° Centigrade (98-4 Fahrenheit) and its fluctuations are fractional. (A moderate and agreeable temperature for a room is about 16°C., or 60° F.) His sensitive body shows signs of distress with a fall or rise of two or three degrees (Fahrenheit) in his internal temperature. A rise of this amount is enough in itself to produce fever symptoms, headache, malaise, and loss of nervous and muscular power. But the essence of the danger of a chilled condition lies in the diminished alertness of the leucocytes. Infections that are quite easily disposed of under normal conditions may now make headway. Colds, influenzas, bacteria always besieging the body, but normally held at bay, get successfully busy as the immediate result of a chill.

The body is continually losing heat to the cooler air which is usually around it,

and therefore it. must have internal sources -

of heat to keep its temperature up. This heat-production is the result of the continual chemical activity of our muscles and glands, for example of the processes needed to keep

our muscles taut and ready for action. The temperature of the body depends upon a balance between this internal heatproduction and the loss of heat from the skin. Both these factors may vary and need toberegulated. The rate of heat-production depends on the activity of our tissues, and of our muscles in particular ; it is very much greater during severe exercise than at rest. The rate of heat-loss depends on the state of the air; on its temperature, motion, and humidity. Therefore, since our own temperatures must be kept strictly constant to secure bodily efficiency, there must be some method of regulating both heat-production and heat-loss, so that the variable factors may be compensated for.

To a certain extent this is done by controlling our own heat-producing processes. Shivering is a way of producing heat by otherwise pointless muscular activity. But such control, since it involves interference with our own life-processes, is inconvenient. The chief part in temperature regulation is played by the skin, which in the case of man is supplemented by clothing.

In most warm-blooded animals the skin is clothed either with hair or feathers. A hair is a rod of dead cells, proliferated from a living root, that sticks out from the skin into the 6uter air. The part that lies inside the skin is housed in a tube, the hairfollicle. Round the mouth of the follicle there is a sensitive nervous belt which we will consider later, and opening into it there are one or two sebaceous glands which secrete a natural brilliantine to keep the hair and skin supple. ‘The outside parts of the hairs form a forest in which imprisoned air stagnates, so that heat loss by convection is reduced. At the same time, since air is a very poor conductor of heat, heat loss by conduction is also diminished. Man, however, does not produce hair or feathers for himself ; he steals them from other creatures or replaces them by woven vegetable fibres.

To a certain extent the insulating power of this air-layer in fur, feathers, or clothing can be varied. In very cold weather a mammal’s fur stands on end and a bird’s feathers are ruffled up in order to increase the thickness of the air jacket. Under the same circumstances a man gets “ gooseflesh? and his hair, such as it is, bristles, which is a feeble attempt to do the same thing. He relies, however, more and more on his hat and overcoat to replace Nature’s failing gifts. ;

The chief mechanisms of heat-regulation are two—variation in size of the skin-

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