THE MUSCLES OF THE THORAX 199 



of one rib (usually the lower ribs), and inserted into the inner 

 surface of the first, second, or third rib below. They are placed 

 on the inner surface of the ribs, where the internal intercostal 

 muscles cease. 



M. Transversus Thoracis Anticus (triangularis sterni). 

 Muscular and tendinous fibers behind the costal cartilages 

 rise from the ensiform, lower part of the sternum, and the 

 cartilages of the lower two or three true ribs; the fibers pass 

 up and out; the lowest are horizontal, the middle oblique, 

 and the upper ones nearly vertical; inserted to the inner sur- 

 faces and lower borders of the sixth to the second costal 

 cartilages, inclusive. It is a continuation upward of the trans- 

 verse oblique muscle; it may be lacking on one side or both 

 sides. 



Gray includes the diaphragm under the muscles of the thorax 

 (see page 194). 



Nerves. All by the intercostal nerve. 



Actions. The external intercostals elevate the rib below. 

 The action of the internal intercostals is not definitely settled 

 Haller early taught that they were accessory muscles of inspira- 

 tion. Others that they are expiratory muscles. During 

 operations for removal of the breast, when they can be seen 

 under forced respiration, their actions seemed to be negative so 

 far as accessory muscles of respiration were concerned. (Little.) 

 Costal and diaphragmatic respiration are normally combined; 

 the thorax is increased anteroposteriorly by a forward move- 

 ment of the sternum, transversely by elevation and eversion 

 of the ribs, vertically by the descent of diaphragm; extension 

 of the vertebral column is also an agent. There are three views 

 as to the action of the intercostals: Hamberger's, that the 

 external elevate and the internal depress the ribs; Hutchinson's, 

 that the external and anterior parts of the internal elevate, and 

 the rest of the internal depress ribs; Haller's, that (1) the ribs 

 are not joined as by a pivot to the vertebral column; (2) are not 

 parallel bars, but curved arches; (3) no two ribs can move as 

 they please, being connected above and below, but all move as 

 a system; if fixed point be above, both the external and internal 

 intercostals elevate the ribs and are inspiratory muscles; fixed 

 below, they both depress and assist expiration. 



