Other Body Structures.--In spaces between the muscles are found various other structures,--blood vessels, which carry blood to and from the great pumping station, the heart, and thence to all parts of the body; connective tissue, which holds groups of muscle or other cells together; fat cells, scattered in various parts of the body; various gland cells, which manufacture enzymes; and the cells of the nervous system, which aid in directing the body parts. Body Cavity.--Within the body is a cavity, which in life is almost completely filled with various organs. A thin wall of muscle called the _diaphragm_ divides the body cavity into two unequal spaces. In the upper space are found the _heart_ and _lungs_, in the lower, the digestive tract with its glands, the _liver_, _kidneys_, and other structures (see page 267). Digestion, Absorption, and Excretion.--Running through the body is a food tube in which undigested food is placed and from which digested or liquid food is absorbed into the blood so that the cells of the various organs which do the work may receive food. Emptying into this food tube are various groups of gland cells, which pour digestive fluids over the solid foods, thus aiding in changing them to liquids. Solid wastes are passed out through the posterior end of the food tube, while liquid wastes are excreted by means of glands called _kidneys_. Work done by Cells.--Food, prepared in the digestive tract, and oxygen from the lungs are taken by the blood to the cells. Bathed in liquid food, the cells do their work; they promote the oxidization of food and the exchange of carbon dioxide for oxygen in the blood, while other wastes of the cells are given off, to pass eventually through the kidneys and out of the body.