_Relations._--Outer surface with the external intercostals (Fig. 69, _i_), and ventrad with the scalenus (Fig. 73, _f_), transversus costarum (Fig. 73, _j_), and rectus abdominis (Fig. 73, _k_). Inner surface with the pleura and the transversus thoracis.
=M. transversus thoracis= (triangularis sterni; sternocostalis internus).--This represents a thoracic portion of the transversus abdominis. It consists of five or six flat muscular bands lying on the inner surface of the thoracic wall.
_Origin_ on the lateral borders of the dorsal face of the sternum, opposite the attachments of the cartilages of the third to the eighth ribs. The six bands thus formed, each about one centimeter wide, pass laterad and are _inserted_ into the cartilages of the ribs near their junction with the ribs, and into the fascia which covers the inner surface of the internal intercostals in this region.
_Relations._--Outer surface with the internal intercostals and the cartilages of the ribs. Inner surface with the pleura.
=Diaphragma= (Fig. 74).--The diaphragm consists of a central so-called =semilunar= tendon (_e_) and of muscular fibres which pass radially from the body wall to the tendon. It forms a complete oblique partition between the abdominal and thoracic cavities. The dorsal end is farther caudad than the ventral.
The central tendon (_e_) is thin and irregularly crescent-shaped, with the convexity ventrad and the horns of the crescent prolonged as two tendinous bands (_e′_) which end in two triangular =membranous= portions (_d_) of the diaphragm, one on each side of the spinal column. It is pierced by an opening for the vena cava (_f_).
[Illustration: FIG. 74.--DIAPHRAGM, CAUDAL SURFACE SEEN OBLIQUELY FROM THE RIGHT.
_a_, right crus; _b_, left crus; _c_, _c′_, sternocostal part of diaphragm; _d_, _d′_, membranous portions of the diaphragm; _e_, central tendon; _e′_, prolongations of central tendon; _f_, opening for posterior vena cava; _g_, œsophagus; _h_, aorta; _i_, M. transversus abdominis.]
The muscular portion is in two parts: (_a_) The =vertebral= portion (_a, b_) arises by a single tendon from the ventral surface of the second, third, and fourth lumbar centra. The tendon diverges into two, the right one (_a_) of which is much stronger, and from each of the two arise muscle-fibres. Each mass of fibres is one of the two =crura= (sing. =crus=) of the diaphragm. The aorta (_h_) enters the abdomen between the crura dorsally. The fibres of each crus diverge to be inserted into the central tendon and the dorsal continuation of its horn. The two sets of fibres unite ventrad of the opening of the aorta. Between this opening and the central tendon is another opening for the œsophagus (_g_); this lies entirely in that part of the diaphragm which arises from the right crus (_a_). The fibres are again united ventrad of the œsophageal opening.