The Pedestrian's Record

2 I O THE PEDESTRIAN'S RECORD. functions under the influence of severe strain. Strength of muscle hasmuch to do with the healthy performance of the respiratory function ; and, therefore,the full muscular develop­ ment ofthe chest must give power bycausing the expansion of the thorax and increasing the breathing faculty. Various muscles in actingupon the bones forming the costal region come into play during respiratory efforts. Within the intervening spaces between each rib thin muscular layers fill up those spaces, andon account of the position they assume are named intercostal muscles. These are the most important for our consideration, as they are th chief agents in effecting the costal movements. The expansion and iminution of the chest is pro­ duced by the separationand approximation of the ribs, " like the rays ofa fan when this is opened and shut." The outer intercostals raise the ribs, and thus advance the sternum so as to increase the diameter of the thoracic cavity. The internal intercostals depress the ribs and draw their lower borders inwards, thusthey diminish the intercostal spaces, and in so doing decreasethe area of the thorax. These muscularactions are for ever occurring during life, and are always actively engaged during the efforts of inspiring pure air and expiring impure gases. Without their aid the function of respiration would cease. There are other muscles than the intercostals which assistthe functionof respiration ; in fact, allthose ofthe chest more or less do so. The pectoralsplay an important part. The pectoralis major is situated on the upper part of the thorax, and arises from the clavicle and front of the sternum, and extends downwards to the cartilages of the ribs. This muscle converges to its insertion on the humerus. The inferior border ofthis musclecovers theserratus magnus ; its posterior surface lies onthe sternum, clavicle, and ribs, and covers the pectoralis minor, subclavius, and serratus magnus. The pectoralisminor arisesfrom the third, fourth, and fifth ribs, close totheir cartilages, and is inserted into the coracoid process, andis joined to the origin ofthe bicepsmuscle.