The various cells of the body are constantly busy, each performing its particular task, either singly or in connection with other cells in the cell-group. Like a great arm, the cells are divided into classes, some being engaged in the active daily work, while others are held back on the reserve line. Some are engaged in building up the tissues, muscles and bones, while others are busy manufacturing the juices, secretions, fluids and chemical compounds required in the great laboratory of the body. Some remain at their posts, stationary during their entire life, while others remain stationary only until the call comes for their services, while a third class are in constant motion from place to place either following regular routes or else travelling under a roving commission. Some of the moving cells act as carriers of material--the hod-carriers of the body, while others move about doing special repair work such as the healing of wounds, etc., while others still are the scavengers and street cleaners of system, and others form the cell army and cell police force. The body has been compared to a vast communistic or socialistic colony, each member of which cheerfully devotes his life-work, and often his life itself, to the common good. The brain cells are of course the most highly organized, and the most highly differentiated of the cells. The nerve cells constitute a living telegraph system over which is carried the messages from the several parts of the body, each cell being in close contact with its neighbor on each side--the nerve cells practically clasp hands and form a living chain of communication.