_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 abdominis; 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 vertebræ (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 tendinous center. The =left crus= (Crus sinistrum) arises by a thin tendon from the inferior common ligament at the first and second lumbar vertebræ. This is succeeded 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 the 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 partially divided into right and left halves by the descent of the crura into it. It is composed largely of radiating fibers, but many interlace in various directions; this is specially evident around the foramen venæ cavæ, which is encircled by fibers. A strong tendinous layer extends across below the hiatus œsophageus. [Illustration: FIG. 185.—DIAPHRAGM OF HORSE, ABDOMINAL SURFACE. _1_, Inferior common ligament; _2_, _2′_, tendons of crura; _3_, lumbar sympathetic trunks; _4_, external spermatic nerve; _5_, _5′_, great splanchnic nerves; _6_, cisterna chyli (opened); _7_, _7′_, œsophageal continuations of vagus nerves; _8_, lymph gland; _9_, coronary ligament of liver (cut); _10_, right lateral ligament of liver (cut); _11_, left lateral ligament of liver (cut); _12_, falciform ligament of liver (cut); _A.l._, lumbo-costal arch; _N.i._, intercostal nerve; _C.d._, right crus; _C.s._, left crus; _A_, aorta; _Ca_, cœliac artery; _Oe._, œsophagus; _V.c._, posterior vena cava; _V.p._, phrenic veins. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]