2. The _elevators of the ribs_, twelve in number on each side, which are so distributed that each single muscle is attached, at one end, to the back portion of a rib and, at the other, to a projection of the vertebra a few inches above. The effect of their contraction is to’ elevate the middle portion of the ribs and to turn them outward or spread them apart. 3. The _intercostal_ muscles, which form two thin layers between the ribs, known as the _internal_ and the _external_ intercostal muscles. The external intercostals are attached between the outer lower margin of the rib above and the outer upper margin of the rib below, and extend obliquely downward and forward. The internal intercostals are attached between the inner margins of adjacent ribs, and they extend obliquely downward and backward from the front. The contraction of the external intercostal muscles raises the ribs, and the contraction of the internal intercostals tends to lower them. [Fig. 42] Fig. 42—*Simple apparatus* for illustrating effect of movements of the ribs upon the thoracic space; strips of cardboard held together by pins, the front part being raised or lowered by threads moving through attachments at 1 and 2. As the front is raised the space between the uprights is increased. The front upright corresponds to the breastbone, the back one to the spinal column, the connecting strips to the ribs, and the threads to the intercostal muscles. By slightly raising and spreading apart the ribs the thoracic space is increased in two directions—from front to back and from side to side. Lowering and converging the ribs has, of course, the opposite effect (Fig. 42). Except in forced expirations the ribs are lowered and converged by their own weight and by the elastic reaction of the surrounding parts. *The Diaphragm.*—Another means of varying the thoracic space is found in an organ known as the diaphragm. This is the dome-shaped, _movable partition_ which separates the thoracic cavity from the cavity of the abdomen. The edges of the diaphragm are firmly attached to the walls of the trunk, and the center is supported by the pericardium and the pleura. The outer margin is muscular, but the central portion consists of a strong sheet of connective tissue. By the contraction of its muscles the diaphragm is pulled down, thereby increasing the thoracic cavity. By raising the diaphragm the thoracic cavity is diminished. The diaphragm, however, is not raised by the contraction of its own muscles, but _is pushed up_ by the organs beneath. By the elastic reaction of the abdominal walls (after their having been pushed out by the lowering of the diaphragm), pressure is exerted on the organs of the abdomen and these in turn press against the diaphragm. This crowds it into the thoracic space. In forced expirations the muscles in the abdominal walls contract to push up the diaphragm.