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. 
Action.—It 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 endothoracie 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.—tThis 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 
vertebre 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 ——(1) 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 sternalis): 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 vertebree. (b) The left crus (Crus sinistrum) is attached in a similar 
fashion to the first and second lumbar vertebre. 
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 cm.) 
therefrom. 
It is stated that the inspiratory movement affects the tendinous center much less than the 
fleshy part, and that the foramen venz cave 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 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 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 Tt should be noted, however, that in the embryo the diaphragm appears as a paired struc- 
ture, extending from the lateral walls of the ceelom to fuse with the septum transversum. 
