322 



THE MUSCLES OF THE TRUNK. 



fibres extend over one or two intercostal spaces. They are most constant on the 

 lower ribs. 



The triangularis sterni (transversus thoracis anterior), a thin stratum of 

 muscular and tendinous fibres placed within the thorax, behind the costal cartilages, 

 arises from the deep surface of the ensiform process and lower part of the body of 

 the sternum, and from the cartilages of two or three of the lower true ribs. Its 

 fibres pass outwards and upwards in a diverging manner, the lowest being horizontal, 

 the middle oblique, and the upper becoming more and more nearly vertical ; they 

 are inserted by separate slips into the outer parts of the cartilages of the ribs, 

 sometimes also into the bones, from the sixth to the second inclusive, on the lower 

 border and inner surface of each. At the lower margin the fibres are in the same 

 plane with those of the transversalis abdominis, of which the triangularis sterni 

 muscle is a continuation upwards. 



Relations. The internal mammary vessels pass between its anterior surface and the costal 

 cartilages : its deep surface is in contact with the pleura, and on the left side also with the 

 pericardium. 



Varieties. The triangularis sterni is subject to much variation as to its extent "and points 

 of attachment in different bodies, and even on the opposite sides of the same body. Occasion- 

 ally it is prolonged to the first rib. Absence on one or both sides has been recorded. 



THE DIAPHKAGM. 



The diaphragm, or midriff, forms a musculo-tendinous partition between the 

 abdominal and thoracic cavities. It consists of fleshy fibres which arch upwards 



Fig. 289. THE LOWER HALF OP THE THORAX, WITH 



FOUR LUMBAR VERTEBRA, SHOWING THE DIA- 

 PHRAGM FROM BEFORE. (Allen Thomson, 

 after Luschka. ) 



a, sixth dorsal vertebra ; &, fourth lumbar 

 vertebra ; c, ensiform process ; d, d', aorta, pass- 

 ing through its opening in the diaphragm ; e, ceso- 

 phagus ; f, opening in the tendon of the dia- 

 phragm for the inferior vena cava ; 1, central, 

 2. right, and 3, left division of the trefoil tendon 

 of the diaphragm ; 4, right, and 5, left costal 

 part, ascending from the ribs to the margins of 

 the tendon ; 6, right, and 7, left crus ; 8, to 8, 

 on the right side, the sixth, seventh, and eighth 

 internal intercostal muscles, deficient towards the 

 vertebral column, where in the two upper spaces 

 the levatores costarum and the external inter- 

 costal muscles 9, 9, are seen ; 10, 10, on the 

 left side, subcostal muscles. 



as they converge from the circum- 

 ference of the visceral cavity to an 

 aponeurotic tendon in the centre, and 

 it is perforated by the various structures 

 which pass from the thorax to the 

 abdomen. 



According to the mode of origin 



and arrangement of its fibres the diaphragm is divided into : a vertebral portion, 

 arising posteriorly from the lumbar vertebrae by two thick processes or crura, and 

 from two fibrous bands on each side external to the bodies of the vertebrae, called 

 arched ligaments; a cosfal portion on each side, arising from the cartilages of 

 the lower six ribs ; and a sternal portion, arising anteriorly from the back of the 

 ensiform process. 



