The first group is intimately connected with the secretory organs, or those organs which secrete certain substances essential for the proper work of the body. Thus we have salivary glands, mucous glands, sweat glands, and the liver and pancreas. Connective tissue includes fibrous tissue, fatty tissue, cartilage and bone. The fibrous connective tissue is illustrated when the skin is easily picked up in folds. Fatty tissue occurs where large amounts of fat are deposited in the cells. Cartilage is found where a large amount of firm support is required. With muscle we are all familiar; it is the real lean meat of the body. =Blood and Lymph.=--The blood is a fluid in which many cells are to be found. The fluid is known as serum or blood-plasma and the cells as corpuscles, and are both red and white. The red cells give the characteristic color. When observed under a microscope, they appear as small, round disks. They are of great importance to the body work. Because of the coloring matter in them the oxygen of the air is attracted when it comes in contact with the blood in the lungs. Oxygen is in reality absorbed, and on the blood leaving the lungs it is distributed to all parts of the body. The oxygen supply of the body is, therefore, in the keeping of the red corpuscles. White corpuscles have a different work; they guard the body by picking up poison, bacteria, and other undesirable elements and cast these out through the natural openings of the body. Compared with the red cells, they exist in far less numbers and may wander about through all parts of the body. Lymph is a fluid in which a few cells, lymph corpuscles, are suspended. These cells are very much like the colorless corpuscles of the blood, only no red blood cells are present. But the lymph attends to its own business; it bathes the tissues and endeavors to keep them in a healthy condition. =Skin and Hair.=--Without a covering the delicate muscles would be unprotected. The skin serves in this capacity. It does still more; out of it is exuded poisonous substances, perspiration, and, at the same time, the skin is a sort of respiratory organ, through which much of the carbonic acid formed in the body escapes. The skin possesses two general layers, the cutis and sub-cutis; in the first is contained also epidermis. Developed in the skin are the outer coverings like hair, wool, feathers, horns, claws, and hoofs. THE FRAMEWORK OF THE BODY The framework of the body undergoes a gradual development from birth to maturity. It represents the bony structure of the body; and on it all other parts depend for support and protection. The brief summary of its parts and work that follows here has been adapted from Wilcox and Smith.