i66 HUMAN ANATOMY. 



the manubrium, but in no case is an entering angle formed in front. A slight twisting 

 may also occur in this joint in the young. In these motions the second costal 

 cartilages follow the manubrium. The motions at the inconstant joint between the 

 sternal body and the ensiform process are necessarily indefinite ; they appear to 

 consist chiefly of a drawing in of the ensiform. 



Motions in the Costo-Sternal and the Interchondral Joints. — On the 

 dissected preparation the second cartilage can be moved up and down, forward and 

 backward, and circumducted ; these motions, however, are very slight. In the 

 succeeding joints the same motions are more and more free as we descend. The 

 lower cartilages of the true ribs from the fifth to the seventh, or to the eighth, inclu- 

 sive, should the latter meet the sternum, move in a somewhat similar manner, but 

 nearly as in one piece. The motion on an antero-posterior axis is most free. The 

 joints between the costal cartilages are very lax, and the surfaces are so placed that 

 the lower one slides forward on the upper. The advantage of these joints is that 

 the lower ribs and the thorax give and receive support, while greater freedom of 

 motion is possible than would be the case were they of one piece. Flexion and 

 extension of the spine modify these motions. The more the spine is flexed the more 

 the upper ribs in particular are depressed, and the more it is extended the more they 

 are raised, independently of any motion in the joints. Thus, when the chest is 

 fully inflated the spine is always strongly extended. 



The elasticity of the ribs and cartilages, particularly of the latter, exercises an 

 important, but indefinite, influence on all motions which does not admit of accurate 

 analysis. Even the ribs (except in the old) are not rigid bars, and, especially in 

 forced inspiration, there is a pull upon them increasing their convexity. Moreover, 

 the walls of the chest adapt themselves to the surface of the lungs and to abnormal 

 contents of the thorax, so that certain conditions are marked by particular forms of 

 thorax. 



It follows from the above that the nature of the respiratory movements cannot 

 be deduced solely from the movements of each set of joints considered separately. 

 The soft parts connecting them alone modify greatly the freedom of motion. Braune 

 has shown that the motion of the ribs is much limited by the sternum, and that if 

 the gladiolus be divided into its original pieces and the cartilage above it cut through, 

 the thorax can be more fully inflated. Beyond question in forced inspiration the 

 sternum is raised, thus increasing the antero-posterior diameter ; since the ribs at 

 the same time swing upward and outward, the transverse diameter is likewise 

 increased. 



Surface Anatomy. — The sternum is always to be felt in the middle line. 

 The suprasternal notch is filled up to a large extent by the interclavicular ligament. 

 The angle between the manubrium and the body varies considerably, but it is always 

 easily recognized by a cross-ridge. The ensiform cartilage is at a deeper level and 

 overhung on each side by the costal arc]a. The front of the chest on each side is 

 covered by the pectoralis major, making it hard to feel the ribs, except at the borders 

 of the sternum. At the side they are easily felt to near the top of the axilla, where 

 the third can be recognized. 



The upper ribs are concealed by thick muscles, especially between the spine 

 and the angles. The scapula covers them from the second to the seventh, with 

 considerable variations. The first rib cannot be felt except where its cartilage 

 joins the sternum. To count the ribs, begin with the second at the junction of the 

 manubrium and body of the sternum. There is no possibility of error, for the rare 

 cases of the manubrium reaching to the third cartilage may be disregarded ; feel the 

 third and fourth cartilages below it, and then carry the finger downward and out- 

 ward across the chest. The twelfth rib may be too small to be made out. It is not 

 safe to begin counting from below, for the error of mistaking the eleventh rib for 

 the twelfth has led to opening the pleural cavity in an operation in the lumbar 

 region. The nipple is said to be usually over the fourth intercostal space some two 

 centimetres external to the cartilage, but it is very variable, especially in women, 

 and should never be used as a starting-point for counting the ribs. The width 

 of the intercostal spaces at different parts is of obvious importance, but has been 

 described elsewhere (page 164). 



