Before describing the movements of the thorax as a whole, it must be premised that there are some few modifications in the movements of certain ribs resulting from their shape. Thus, the firs rib (and to a less extent the second also), which is flat on its upper and lower surfaces, revolves on a transverse axis drawn through the costo-vertebral and costo-transverse joints. During inspiration and expiration, the anterior extremities of the first pair of costal arches play up and down, the tubercles and the heads of the ribs acting in a hinge-like manner, the latter having also a slight screwing motion. By this movement, the anterior ends of the costal arches are simply raised or depressed, and the sternum pushed a little forward; it may be likened to the movement of a pump-handle.
The movements of the other ribs, particularly in the mid-region of the thorax, are more complex, for, besides the elevation of the anterior extremities, the bodies and angles of the ribs rise nearly as much as the extremities themselves. In this movement, the tubercles of the ribs glide upward and backward in inspiration, and downward and forward in expiration; and the movement may be likened to that of a bucket handle.
During inspiration, the cavity of the thorax is increased in every direction. The antero- posterior diameter is increased by the thrusting forward of the sternum, caused by the elevation of the costal cartilages and fore part of the ribs, whereby they are brought to nearly the same level as the heads of the ribs. The transverse diameter is increased:
- Behind, by the elevation of the middle part of the ribs; for when at rest the mid-part of the rib is on a lower level than either the costo-vertebral or chondro-sternal articulations. Owing to this obliquity the transverse diameter is increased when the rib is raised, and the increase is proportionate to the degree of obliquity,
- By the eversion of the lower border of the costal arch, which turns outward as the arch is raised,
- The transverse diameter is increased in front by the abduction of the anterior extremity of the rib at the same time as it is elevated and thrust forward.
- The increase in the vertical diameter of the thorax is due to the elevation of the ribs, especially the upper ones, and the consequent widening of the intercostal spaces; but the chief increase in this direction is due to the descent of the diaphragm.
The greatest increase both in the antero-posterior and transverse diameters takes place where the ribs are longest, most oblique, and most curved at their angles, and where the bulkiest part of the lung is enclosed. This is on a level with the sixth, seventh, and eighth ribs.
At the lower part of the thorax, where the ribs have no relation to the lungs, and do not affect respiration directly by their movements, it is important that the costal arches should be thrown well outward in order to counteract the compression of the abdominal viscera by the contraction of the diaphragm.
By widening and steadying the lower part of the thorax during inspiration, the attachments of the muscular fibers of the diaphragm are widened, and their power increased.
Muscles which take part in the movements of inspiration
Ordinary inspiration: The scalenes, serratus posterior superior, the external and internal intercostals, the diaphragm; the quadratus lumborum and serratus posterior inferior fixing the lower ribs, possibly the posterior fibers of the external oblique also helping to fix the lower ribs
Extraordinary inspiration: The superior extremities are raised and fixed. The cervical part of the vertebral column and the head are extended, and in addition to the muscles of ordinary inspiration, the following muscles also come into play: The pectoralis minor, the muscles which extend the head and the cervical part of the vertebral column, the sterno-mastoid and the supra- and infra-hyoid muscles, the lower fibers of the pectoralis major, some of the lower fibers of the serratus anterior, and, when the clavicle is fixed, the subclavius.
Expiration is produced by the elasticity of the lungs and the weight of the thorax, aided by the elastic reaction and contraction of the external and internal oblique muscles, the recti and pyramidales, the transversus abdominis, and the levatores ani and coccygei. In forcible expiration, all muscles which depress the ribs and reduce the dimensions of the abdomen are thrown into action. The internal intercostals probably tend to contract the thorax, excepting, the parts between the costal cartilages, which tend to expand the thorax.