512 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



In considering the interosseous portion of the internal intercostals we find 

 that during the passive condition they are placed nearly at right angles to the 

 ribs. If contraction takes place, it is obvious that the mechanical response 

 must be an approximation of the ribs and a lessening of the width of the inter- 

 costal spaces. It must also be apparent that during the movement of inspira- 

 tion these fibres are put on the stretch, which can be demonstrated in the above 

 model. Thus, if we put a rubber band at right angles to the parallel rods 

 (Fig. 135), we will find that when the rods are in the horizontal position, in 

 imitation of the position of the ribs at the beginning of expiration, the distance 

 between c, d is greater than that between a, b ; therefore if we lessen the dis- 

 tance between c, d f as when the muscle-fibres contract, the mechanical result of 

 contraction must be approximation, the opposite to that which occurs during 

 inspiration. 



While the whole subject of the actions of the intercostal muscles must still 

 be regarded as in an unsettled condition, yet there is no reasonable doubt that 

 the externi and the intercartilaginei contract during inspiration, and the inter- 

 osseous portion of the internal intercostals during expiration. Admitting this 

 to be true, it is, however, by no means clear whether or not these muscles are 

 for the purpose of altering the volume of the thorax. It is probable, as sug- 

 gested by Landois, that their chief function is to maintain, during all phases 

 of the respiratory movements, a proper degree of tension of the intercostal 

 tissues. If this view be correct, the external intercostals and interchondrals con- 

 tract during inspiration chiefly for the purpose of causing greater tension of 

 the intercostal tissues, so as to counteract the influence of the increase of 

 negative intrathoracic pressure; while during expiration, when their relax- 

 ation occurs, a substitution for this relaxation is provided by the contraction 

 of the interosseous portion of the internal intercostals, so that the tension of 

 the intercostal tissues is maintained. The internal intercostals must prove 

 most effective during forced expiratory efforts for example, in coughing, 

 when the intercostal tissues are subjected to high positive intrathoracic pres- 

 sure, and there is a consequent tendency to outward displacement, which is 

 met and counteracted by the internal intercostals. 



During forced inspiration the scaleni and the serrati postiei superiores con- 

 tract vigorously, so that the sternum and the first five ribs are elevated, thus 

 raising the thoracic cage as a whole. At the same time the serrati postid 

 inferiores, the quadrati lumborum, and the sacr-o-lumbales are active in pulling 

 the lower ribs downward and backward. Besides these muscles there are a 

 number of others which directly or indirectly affect the size of the thorax and 

 which may be brought into activity ; chief among these are the sterno-deido- 

 mastoidei, the trapezei, the pectorales minores, the pectorales majores (costal 

 portion), the rhomboidei, and the erectores spince. 



The sterno-cleido-mastoid passes from the mastoid process and the superior 

 curved line of the occipital bone to the upper front surface of the manubrium 

 and the upper border of the inner third of the clavicle. These muscles ele- 

 vate the upper part of the chest when the head and neck are fixed. The 



