308 MUSCLES OF THE THOEAX. 



The levatores costarum, t"^elve on each side, arise from the tips of 

 the transverse processes of the seventh cervical and eleven highest 

 dorsal vertebrae. Corresponding in direction with the external inter- 

 costal muscles, on which they lie, they pass downwards and outwards, 

 spreading as they descend, and each is inserted into the outer surface of 

 the rib belonging to the vertebra below that from which it springs. 

 The levator muscles belonging to the lower ribs present some longer 

 additional fibres which, passing over a rib, are inserted into the next 

 one below ; these fibres are sometimes distinguished as levatores cos- 

 tarum Jongiorcs. 



Rclntionx. — The levatores costaram lie in series superiorly with the scaleni 

 medius and posticus, and inferiorly with the lateral lumbar intertransverse 

 muscles. 



The subcostal muscles are small bundles lying on the inner aspect 

 of the th.oracic wall, close to the surface of the internal intercostals, 

 and chiefly in the neighbourhood of the angles of the ribs. They 

 follow the same direction as the internal intercostal muscles, but their 

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

 constant on the lower ribs (see fig. 221). 



The triangularis sterni, 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 of the lower part of the body of 

 the sternum, and of the cartilages of one or two of the lower true ribs. 

 Its fibres pass outward 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 carti- 

 lages of the true ribs from the sixth to the second inclusive, on the 

 lower border and inner surface of each, at the junction with the bony 

 part. At the lower margin the fibres are in the same plane with those 

 of the transversalis abdominis, of wliich the triangularis sterni muscle 

 is a continuation upwards. 



liclaflonft. — Tlie internal mammary ai-tery and veins pass between its anterior 

 surface and the costal cartilages ; the plem-a is m contact with its deep surface. 



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. The subcostales also vary greatly in then- extent in different 

 subjects. 



The rectus sternnHs, or sternalis Irutornm, is an elongated muscle, 'of nearly 

 the same length as the sternum, frequently seen lying in front and i^arallel to the 

 outer margin of that bone, and over the inner part of the pectoral muscle. It is 

 fleshy in the middle and tendinous at both ends ; it is attached superiorly to the 

 sternum in connection with the tendon of the sterno-mastoid, and inferiorly it is 

 connected with the rectus abdominis muscle. It is rarely present on both sides. 

 It represents a muscle which is constant in some animals. 



THE DIAPHEAGM. 



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

 between the abdominal and thoracic cavities. It consists of muscular 

 fibres which arch upwards as they converge from the circumference of 

 the visceral cavity to a tendinous structure in the centre, and it is 

 perforated by the various structures which pass from the thorax to the 

 abdomen. 



