The third kind of supporting material is connective tissue. This is composed of tiny threads or fibers, some of which are inelastic, others are very elastic. The inelastic fibers are found in places where a flexible, but unyielding support is required; the elastic fibers are located where elasticity is particularly important. Either kind of fiber may be grouped into sheets, or into loose networks, or into stout cords. A good example of inelastic connective tissue in sheet form is in the _mesentery_ which holds the organs of the abdominal cavity in place. Just under the skin, anchoring it loosely to the underlying muscles, is connective tissue in network form. The tendons by which most of the muscles are attached to the bones upon which they pull are made up of inelastic connective tissue in the form of cords. The best example of elastic connective tissue is in the large arteries, which are just as elastic as best quality rubber tubing. Another good example is the large and strong elastic cord which passes along the back of the neck in cattle and sheep, and helps to support the weight of the head. Connective tissue fibers are deposited by living connective tissue cells. Since connective tissue is of open and relatively loose construction, there is no problem presented in supplying the cells with material. The meshes among the fibers are filled with fluid, and this fluid has ready connection, in turn, with the blood. Use is made of the abundance of body fluid in the connective tissue spaces whenever a subcutaneous injection is given, for what is done is to inject the desired material into the fluid which fills the spaces in the connective tissue just beneath the skin, trusting that it will pass from there to the blood, which it does rather gradually, and so is distributed about the whole body. While we are on the topic of the supporting framework, something must be said about the grouping of the bones into what we know as the skeleton. Of course it is evident that the effectiveness of the bony part of the framework depends almost altogether on the way in which the individual bones are grouped together. If the whole skeleton were [Illustration: THE BONY AND CARTILAGINOUS SKELETON (From Martin’s “Human Body”)]