THE MUSCLES OF THE THORAX 285 



6. Transversus thoracis.— This is a flat muscle situated on the thoracic sur- 

 face of the sternum and the cartilages of the sternal ribs. 



Origin. — The sternal ligament, meeting the opposite muscle. 



Insertion.— The cartilages of the ribs, from the second to the eighth inclusive, 

 and the adjacent part of some of the ribs. 



_ Adion.—lt draws the ribs and costal cartilages inward and backward, thus 

 assisting in expiration. 



Structure. — Each muscle has the form of a scalene triangle, of which the base 

 is the strongly serrated lateral border. The muscle contains a good deal of ten- 

 dinous tissue. The anterior bundles are directed forward and outward; the poste- 

 rior, backward and outward. 



Relations. — Dorsally, the endothoracic fascia and pleura; ventrally, the 

 costal cartilages, the internal intercostal muscles, and the internal thoracic vessels. 



Blood-supply. — Internal thoracic artery. 



Nerve-supply. — The intercostal nerves. 



7. Diaphragm. — This is a broad, unpaired muscle which forms a partition be- 

 tween the thoracic and abdominal cavities.^ In outline it has some resemblance to 

 a palm-leaf fan. In form it is dome-shaped, compressed laterally. On a median 

 section it is seen to have a general direction downward and forward from the lumbar 

 vertebrae to the xiphoid cartilage. The thoracic surface is strongly convex, and 

 is covered by the pleura. The abdominal surface is deeply concave, and is covered 

 for the most part by the peritoneum. The muscle consists of a fleshy rim which 

 may be subdivided into costal and sternal parts; a lumbar part, composed of two 

 crura; and a tendinous center. 



Attachments. ~{l) Costal part (Pars costalis) : The cartilages of the eighth, 

 ninth, and tenth ribs, and behind this to the ribs at an increasing distance from their 

 sternal ends. 



(2) Sternal part (Pars sternahs) : The upper surface of the xiphoid cartilage. 



(3) Lumbar part (Pars lumbalis) : (a) The right crus (Crus dextrum) is at- 

 tached to the ventral longitudinal ligament, and by this means to the first four or 

 five lumbar vertebrae. (5) The left crus (Crus sinistrum) is attached in a similar 

 fashion to the first and second lumbar vertebrae. 



Action. — It is the principal muscle of inspiration and increases the longitu- 

 dinal diameter of the chest. The contraction produces a general lessening of the 

 curvature of the diaphragm. In the expiratory phase the costal part and crura lie 

 almost entirely on the body walls, so that the bases of the lungs are in contact with 

 the tendinous center almost exclusively. In ordinary inspiration the fleshy rim 

 recedes from the chest-wall, so that the bases of the lungs move backward to a line 

 about parallel with the costal arches, and about four or five inches (ca. 10-12 era.) 

 therefrom. 



It is stated that the inspiratory movement affects the tendinous center much less than the 

 fleshy part, and that the foramen venae cavaj scarcely moves at all, since the posterior vena cava 

 is firmly attached to it. It should be noted, however, that the direction of the thoracic part of 

 the vena cava in the expiratory phase is obhque upward and backward. Thus it would seem that 

 there is no anatomical reason why the diaphragm should not move as a whole in ordinary inspira- 

 tion at least; examination of formalin-hardened subjects in which the diaphragm appears to be 

 fixed in the inspiratory phase indicates that such is the case. 



Structure. — The costal part consists of a series of digitations which meet, or 

 are separated by a very narrow interval from, the transversus abdominis; between 

 the two are the musculo-phrenic vessels. From the tenth rib backward the at- 

 tachments to the ribs are at an increasing distance above the costo-chondral junc- 

 tions. Thus at the last rib the upper limit of the attachment is four to five inches 

 (10-12 cm.) from the ventral end. Anteriorly the origin extends along the eighth 



1 It should be noted, however, that in the embryo the diaphragm appears as a paired struc- 

 ture, extending from the lateral walls of the ooelom to fuse with the septum transversum. 



