We have muscle cells, with long, thin bodies like pea-pods, who devote their lives to the business of contraction; thin, hair-like connective tissue cells, whose office is to form a tough tissue for binding the parts of the body together; bone cells, a trades-union of masons, whose life work it is to select and assimilate salts of lime for the upkeep of the joints and framework; hair, skin, and nail cells, in various shapes and sizes, all devoting themselves to the protection and ornamentation of the body; gland cells, who give their lives, a force of trained chemists, to the abstraction from the blood of those substances that are needed for digestion; blood cells, crowding their way through the arteries, some making regular deliveries of provisions to the other tenants, some soldierly fellows patrolling their beats to repel invading disease germs, some serving as humble scavengers; liver cells engaged in the menial service of living off the waste of other organs and at the same time converting it into such fluids as are required for digestion; windpipe and lung cells, whose heads are covered with stiff hairs, which the cell throughout its life waves incessantly to and fro; and, lastly, and most important and of greatest interest to us, brain and nerve cells, the brain cells constituting altogether the organ of objective intelligence, the instrument through which we are conscious of the external world, and the nerve cells serving as a living telegraph to relay information, from one part of the body to another, with the "swiftness of thought." Says one writer, referring to the cells of the inner or true skin: "As we look at them arranged there like a row of bricks, let us remember two things: first, that this row is actually in our skin at this moment; and, secondly, that each cell is a living being--it is born, grows, lives, breathes, eats, works, decays and dies. A gay time of it these youngsters have on the very banks of a stream that is bringing down to them every minute stores of fresh air in the round, red corpuscles of the blood, and a constant stream of suitable food in the serum. But it is not all pleasure, for every one of them is hard at work." [Sidenote: Cell Life After Death] And again, speaking of the cells that line the air-tubes, he says: "The whole interior, then, of the air-tubes resembles nothing so much as a field of corn swayed by the wind to and fro, the principal sweep, however, being always upwards towards the throat. All particles of dust and dirt inhaled drop on this waving forest of hairs, and are gently passed up and from one to another out of the lungs. When we remember that these hairs commenced waving at our birth, and have never for one second ceased since, and will continue to wave a short time after our death, we are once more filled with wonder at the marvels that surround us on every side." [Sidenote: Experiments of Dr. Alexis Carrel]