THE MUSCLES OF THE THORAX 243 



Relations. — Superiorly, the endothoracic fascia and pleura; inferiorly, the 

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



Blood-supply. — Internal thoracic artery. 



Nerre-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 lateral!}'. 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 peripheral fleshy 

 portion, two muscular crura, and a tendinous center. 



Attachments. — (1) Costal part: The cartilages of the ninth to the fifteenth 

 ribs, and the last three ribs at an increasing distance from their sternal ends. 



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



(3) Lumbar part: (a) The right cms is attached to the inferior common 

 ligament, and by this means to the first four or five lumbar vertebra^, (h) The left 

 crus is attached in a similar fashion to the first and second lumbar vertebrae. 



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

 tudinal 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 directly on the body walls, so that the bases of the lungs are in 

 contact with the tendinous center and sternal portion only. In ordinary inspira- 

 tion the fleshy rim recedes from the chest -wall, so that the bases of the lungs move 

 backward to a line about parallel with the curve formed by the cartilages of the 

 asternal ribs, and about four or five inches (ca. 10 to 12 cm.) therefrom. It is 

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

 the fleshy part, since the posterior vena cava is firmly attached to the former. It 

 should be noted, however, that the direction of the thoracic part of the vena cava 

 in the expiratory phase is oblique upward and backward. Thus it would seem 

 that there is no anatomical reason why the diaphragm should not move as a whole 

 in ordinary inspiration at least. 



Structure. — The costal part (Pars costalis) consists of a series of digitations 

 which meet, or are separated by a very narrow interval from, the transversus ab- 

 dominis; between the two are the asternal vessels. From the tenth rib backward 

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

 junctions. Thus at the last rib the attachment is four to five inches (10 to 12 cm.) 

 from the lower end. Anteriorly, the origin extends along the ninth costal cartilage 

 to the xiphoid cartilage. From these points of origin the fibers curve inward and 

 forward to join the tendinous center. The right crus (Crus dextrum) is about twice 

 as thick as the left one and is also longer. It arises by a strong tendon from the 

 lumbar vertebrae (by means of the inferior common ligament). The tendon is 

 succeeded by a rounded belly which leaves the vertebral column, at the last thoracic 

 vertebra. Passing downward and forward, its fibers spread out and join the tend- 

 inous center. The left crus (Crus sinistrum) arises by a thin tendon from the 

 inferior common ligament at the first and second lumbar vertebrae. This is suc- 

 ceeded by a triangular belly which joins the central tendon. Between the 

 crura and the attachment to the last rib the edge of the muscle crosses the ventral 

 surface of the psoas muscles without attachment, forming th^ so-called lumbo- 

 costal arch; here the thoracic and abdominal cavities are separated only by the 

 serous membranes and some areolar tissue. The tendinous center (Centrum 

 tendineum) resembles the periphery in outline, but is more elongated. It is par- 



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 coelom to fuse with the septum transversum. 



