OF THE THORAX. 289 



intercostal vessels and nerves. By their internal surface, with the pleura costalis. 

 Triangularis sterni, and Diaphragm. 



The Intercostal muscles consist of muscular and tendinous fibres, the latter 

 being longer and more numerous than the former ; hence these spaces present very 

 considerable strength, to which their crossing materially contributes. 



The Infra-costaks consist of muscular and aponeurotic fasciculi, which vary in 

 number and length ; they arise from the inner surface of one rib, and are inserted 

 into the inner surface of the first, second, or third rib below. Their direction 

 is most usually oblique, like the Internal intercostals. They are most frequent 

 between the lower ribs. 



The Triangularis Sterni is a thin plane of muscular and tendinous fibres, 

 situated upon the inner wall of the front of the chest. It arises from the lower 

 part of the side of the sternum, from the inner surface of the ensiform cartilage, 

 and from the sternal ends of the costal cartilages of the three or four lower true 

 ribs. Its fibres diverge upwards and outwards, to be inserted by fleshy digitations 

 into the lower border and inner surfaces of the costal cartilages of the second, 

 third, fourth, and fifth ribs. The lowest fibres of this muscle are horizontal in 

 their direction, and are continuous with those of the Transversalis ; those which 

 succeed are oblique, whilst the superior fibres are almost vertical. This muscle 

 varies much in its attachment, not only in different bodies, but on opposite sides 

 of the same body. 



Relations. In front, with the sternum, ensiform cartilage, the costal cartilages, 

 the Internal intercostal muscles, and internal mammary vessels. Behind, with 

 the pleura, pericardium, and anterior mediastinum. 



The Levatores Costarum, twelve in number on each side, are small tendinous 

 and fleshy bundles, which arise from the extremities of the transverse processes 

 of the dorsal vertebrae, and, passing obliquely downwards and outwards, are in- 

 serted into the upper rough surface of the rib below them, between the tubercle 

 and the angle. That for the first rib arises from the transverse process of the last 

 cervical vertebra, and that for the last from the eleventh dorsal. The inferior 

 Levatores divide into two fasciculi, one of which is inserted as above described ; the 

 other fasciculus passes down to the second rib below its origin : thus, each of the 

 lower ribs receives fibres from the transverse processes of two vertebrae. 



Nerves. The muscles of this group are supplied by the intercostal nerves. 



Actions. The Intercostals are the chief agents in the movement of the ribs in 

 ordinary respiration. The External intercostals raise the ribs, especially their fore 

 part, and so increase the capacity of the chest from before backwards ; at the same 

 time they evert their lower borders, and so enlarge the thoracic cavity trans- 

 versely. The Internal intercostals, at the side of the thorax, depress the ribs, and 

 invert their lower borders, and so diminish the thoracic cavity ; but at the fore 

 part of the chest these muscles assist the External intercostals in raising the car- 

 tilages. The Levatores Costarum assist the External intercostals in raising the 

 ribs. The Triangularis sterni draws down the costal cartilages ; it is therefore an 

 expiratory muscle. 



DIAPHRAGMATIC EEGIOX. 



Diaphragm. (Fig. 167.) 



The Diaphragm (Aia}>pa<j<j, to separate two parts], is a thin musculo-fibrous sep- 

 tum, placed obliquely at the junction of the upper with the lower two-thirds of 

 the trunk, and separating the thorax from the abdomen, forming the floor of the 

 former cavity and the roof of the latter. It is elliptical, its longest diameter being 

 from side to side, somewhat fan-shaded, the broad elliptical portion being horizon- 

 tal, the narrow part, which represents the handle, being vertical, and joined at right 

 angles with the former. It is from this circumstance that some anatomists describe 

 it as consisting of two portions, the upper or greater muscle of the Diaphragm, 

 and the lower or lesser muscle. It arises from the whole of the internal circum- 

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