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 Changes in Blood in the Kidneys.—The water holding the nitrogenous waste varies in amount with the amount of water drunk and with the activity of the skin, being less in summer when the perspiration is great. The lungs aid the skin and kidneys in disposing of superfluous moisture. The kidneys have almost the entire responsibility of relieving the body of certain mineral salts and a white crystalline solid called urea. This is very injurious if retained, causing headaches, rheumatism, and other troubles.

, Tb, with artery A, and V in pV.

Hygiene of the Skin.—1. What kind of a scar is not affected by freckles or tan? 2. Can a scar on a negro be white? 3. Does a scar on a child grow in size? 4. Why is heat most oppressive in moist weather? 5. How do you account for the shape and location of the usual bald spot? 6. How does the wearing away of the outer cells of the epidermis contribute to the cleanliness of the body? 7. Why does the palm of the hand absorb water more rapidly than the back of the hand? 8. Is it more necessary for mental workers to bathe often or change their clothes often? For physical workers? 9. Is cotton or woolen clothing more liable to stretch or shrink out of shape or size? To catch fire? To make the skin clammy with moisture? To cost more? To be eaten by moths?