The anterior extremities of the ribs are generally on a much lower level than their posterior. Thus, the anterior end of the first rib is on a level with the head of the fourth, the second with the sixth, the third with the seventh, and the seventh with the eleventh. The first rib is the shortest, the seventh the longest, and the ninth the most oblique. In counting the ribs it is useful to remember that the manubrio-gladiolar junction denotes the position of the second costal cartilage. The male nipple is generally situated between the fourth and fifth ribs. When the arm is raised, the first visible digitation of the serratus magnus corresponds to the sixth rib. The scapula covers the second to the seventh rib inclusive, and when the arm is at rest and applied to the side, its angle reaches the level of the eighth rib. The twelfth rib is occasionally absent, and, on the other hand, accessory ribs may be present. These generally occur in the lumbar or cervical region. The latter are generally met with in connection with the seventh cervical vertebrae, and sometimes give rise to trouble from irritation of the cords of the brachial plexus, necessitating their removal.

The ribs are very elastic, and this quality is increased in young persons by the elastic costal cartilages, and hence they frequently escape fracture, even when the violence is sufficient to damage the underlying organs. As a rule, fractures of ribs occur in older persons. Where fracture is caused by direct violence, the pleura is apt to be injured by driving in of the fragments; whereas in fracture from indirect violence, where the fracture frequently occurs between the angle and the centre of the bone, the fragments tend to be driven out, and the pleura generally escapes. In addition to the pleurae, the lungs, heart, and even the liver and spleen may be damaged in severe cases of fracture. The ribs most frequently fractured are the sixth, seventh, and eighth, while the first is very rarely fractured, and the second and third, and also the fourth, eleventh, and twelfth generally escape.

As a rule, the fracture is not attended with displacement, save in severe injuries, where several ribs have been damaged, and possibly driven in. Ribs have been occasionally fractured by muscular violence, as in sneezing and coughing, but probably in such cases have been affected by disease-as, for example, tubercle-which very frequently affects the ribs (insane rib), giving rise to superficial cold abscesses, which, owing to the laxness of the subcutaneous tissues, may attain a considerable size.

The rickety rosary is a bilateral enlargement of the ribs at the costo-chondral junction, due to rickets.

The costal cartilages increase in length up to the seventh or eighth, and then again decrease, and the lower cartilages run obliquely upwards from the ribs to the sternum. The cartilages are occasionally fractured in older persons by direct violence, producing a sharp transverse fracture, which generally heals by tissue containing osseous matter.

The intercostal spaces are wider in front than behind, and increase in breadth from above downwards. The widest space is the third, then the second and first. They are-increased on inspiration, by emphysema, pleural effusions, etc. When collapse of the lung has occurred, on the other hand, the ribs become flattened and crowded together, while the diaphragm ascends.

Paracentesis (tapping of the chest), or thoracotomy (incision into the chest through an intercostal space), is generally performed in the sixth or seventh space and in the midaxillary line, the instrument being entered during inspiration as close to the upper border of the lower rib as possible, so as to avoid the intercostal vessels. Tapping should not be performed behind the angles of the ribs, owing to the thick layer of muscles, and owing to the oblique course of the intercostal artery, nor should it be performed through the lower spaces, owing to the danger of wounding the diaphragm. When done in the eighth or ninth space, the puncture should be made just in front of the line of the angle of the scapula. Where more space is required, a portion of one or more ribs may be excised subperiosteally, so as to avoid damage to the vessels.

The intercostal spaces are occupied by the intercostal muscles, which consist of an external and internal layer. The fibres of the external set, like those of the external oblique of the abdomen, run downwards and forwards, and extend from the tubercles of the ribs posteriorly to the costo-chondral junctions anteriorly, beyond which a membranous layer extends forwards to the sternum. The fibres of the internal layer, like those of the internal oblique, run at right angles to those of the external, and extend from the sternum to the angles of the ribs, beyond which a membranous layer extends backwards to the vertebrae. Pus arising from disease of the vertebrae, or adjoining portions of ribs, may be conducted along between these muscular layers, and thus present anteriorly. A thin layer of connective tissue (the endo-thoracic fascia) exists between the ribs and intercostal muscles and the parietal pleura.

The intercostal arteries are given off from the aorta, with the exception of the first two, which arise from the superior intercostal of the subclavian. The arteries of the right side cross the front of the vertebral column behind the oesophagus, thoracic duct, and vena azygos major. Each runs out and backwards on the side of the vertebral body to the intercostal space behind the pleura and sympathetic cord, and then ascends to the lower border of the rib forming the upper boundary of the space, whence it runs forward in the subcostal groove, at first between the pleura and the posterior intercostal membrane, and then, having pierced the membrane, between it and the external intercostal muscle, and, farther out, between the two muscles. In the groove the artery is accompanied by the vein, which lies above it, and the nerve, which lies below it. At the anterior end of the space it anastomoses with the intercostal branches of the internal mammary. The third, fourth, and fifth intercostal arteries give on mammary branches, which supply the breast, and become enlarged during lactation.

The internal mammary artery arises from the subclavian artery at the inner border of the scalenus anticus, and runs downwards, inwards, and forwards, the phrenic nerve crossing it in front, to the under surface of the first costal cartilage, and then runs downwards about inch beyond the border of the sternum, in front of the pleura and triangularis sterni muscle. It anastomoses with the intercostal arteries, and with the artery of the other side, and sends off perforating branches to the pectoralis major and mammary gland, that in the second space being generally the largest, and ends in the musculophrenic artery, which supplies the diaphragm, and the superior epigastric, which anastomoses with the deep epigastric. These anastomoses are of importance, as they come into play after ligature of the subclavian or axillary, and common or external iliac vessels. The internal mammary is most easily ligatured through the second intercostal space.

The anterior mediastinal lymphatic glands, generally two in number for each intercostal space, lie round the artery. They receive lymph from the diaphragm, anterior portions of the intercostal spaces, and inner portion of the mammary gland. This latter communication should be remembered in cases of extensive tubercular or carcinomatous affections of the breast, and the possibility of involvement of these glands considered. While the five upper intercostal nerves supply the chest-wall and integument (the intercosto-humeral of the second supplying the skin on the inner and posterior aspects of the upper two-thirds of the arm), the lower six extend to the abdomen, supplying skin, muscles, and parietal peritoneum. Painful affections of these latter nerves, therefore, frequently give rise to abdominal tenderness and rigidity, which may be mistakenly regarded as arising from peritonitis, and somewhat similar symptoms are not infrequently manifested in the early stages of spinal caries, the patient referring the pain to the region of the umbilicus. A painful affection of the upper intercostal nerves is termed ' intercostal neuralgia,' and may be followed by herpes along the course of the nerve (herpes zoster, or shingles). The triangularis sterni muscle rises from the deep surface of the lower portion of the sternum, xiphoid, and fifth, sixth, and seventh costal cartilages, and is inserted into the deep surfaces of the second to fifth costal cartilages.