PRACTICAL CONSIDERATIONS : THE THORAX. 169 



the posterior surface of the sternum, as compared with that on the anterior surface, 

 account for the rarity with which effusions of blood or collections of purulent fluid 

 find their way to the anterior mediastinum. 



The ribs, in addition to the already described classification into sternal, 

 asternal, and floating, are sometimes designated as upper and lower. It may be 

 well to mention that the term ' ' upper' ' includes the first six ribs, which have convex 

 lower borders, give origin to the pectoralis major (an elevator of the ribs), and move 

 upward in inspiration ; while the term "lower" applies to the last six ribs, which 

 have concave lower borders, give origin to the diaphragm (a depressor of the ribs), 

 and move downward in inspiration. 



The obliquity of the ribs adds greatly to their range of movement in respiration. 

 The most oblique rib, the longest, and the most movable — the seventh — is a part 

 of the wall of that portion of the thorax that contains the largest amount of pulmo- 

 nary tissue. The most fixed and most nearly horizontal of the ribs (and the shortest 

 of the sternal ribs) — the first — is a part of the wall where the least lung tissue is to 

 be found. The ribs below the eighth have less and less relation to the lungs, and 

 become both shorter and more horizontal. They have increased mobility as regards 

 their anterior ends, but lessened rotation on a line drawn between their two extrem- 

 ities, the movement most important in respiration. 



These facts have relation to the distribution of acute and chronic disease in 

 the lungs : the acute affecting particularly the area of greatest movement and vas- 

 cularity, the bases ; the chronic, the area of lessened mobility and expansion, the 

 apices. 



The involuntary partial immobilization of the chest-wall after injury and in 

 inflammatory affections of the pleura is of some diagnostic value, as is also the 

 permanent restriction of its movements following the contraction of old adhesions, 

 as after a pleurisy, or pleuro-pneumonia, or fibroid phthisis. 



The obliquity of the ribs serves also the purpose of securing the necessary 

 expansion of the chest with the least possible motion in the joints between the ribs 

 and the spine and between the cartilages and the sternum. They are thus but little 

 liable to strain, and, in spite of their unceasing movement during life, are very rarely 

 the seat of either dislocation or disease. 



At the articulation of the ribs with the spine the provision for preventing the 

 ascent of the ribs during the action of the inspiratory muscles (similar to that at the 

 costo-sternal junction) is seen in the fact that the articulating surface of the upper 

 vertebra entering into the joint stands out more boldly than that of the lower one. 

 The participation of the intervertebral disks in the costo-vertebral articulation gives 

 greater safety to those joints and adds to the elasticity of the whole thorax by 

 furnishing a resilient buffer which takes up and distributes forces directed against 

 the chest-wall. 



Variation in the development of the costal element of the seventh cervical 

 vertebra (page 129) may result in the production of a cervical rib. This, growing 

 beyond its ordinary limits, sometimes reaches half-way to the sternum, running 

 parallel to the first rib, with which its anterior end is sometimes joined. Occasion- 

 ally a process grows up from the first rib to meet it. This, or the cervical rib itself, 

 may raise the subclavian artery and give rise to a mistaken diagnosis of aneurism, 

 or may be thought to indicate chronic (tuberculous or syphilitic) infection of bone, 

 and lead to unnecessary operation or treatment. 



As a result of rickets, changes often take place at the chondro-costal junctions, 

 causing beaded ribs when a few bones only are affected, or the "rickety rosary" 

 when the enlargements are bilateral and numerous. 



The ribs most frequently broken are the sixth, seventh, and eighth ; the first 

 and second are protected by the clavicle ; the lower two by their small size and great 

 mobility. The most common form of muscular action causing fracture is coughing ; 

 sneezing and lifting heavy weights have had the same effect. The lower ribs are 

 most frequently broken in this way. When the first rib is broken, a character- 

 istic symptom is said to be pain behind the upper part of the sternum on lifting 

 with the hand on the injured side. This may be due to the fact that the first 

 thoracic nerve lies for about two inches in contact with the under surface of the first 



