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. (_b_) The =sternocostal part= (_c_, _c′_) takes origin from the xiphoid process and the last five ribs, by fleshy bundles which interdigitate with those of the transversus abdominis (_i_). The fibres converge to the central tendon (_e_). Between the crus of each side and the most dorsal of the costal fibres is the membranous interval (_d_) mentioned above. 4. =Abdominal Muscles.= =M. obliquus abdominis externus= (Fig. 68, _p_).--A large, thin sheet of muscle covering the whole abdomen and part of the thorax ventrally. _Origin._--(_a_) From the last nine or ten ribs by means of as many tendons, which are interconnected to form arches that span the slips of the serratus anterior. The muscle-fibres arise from these tendons and from their intervening arches. (_b_) From the lumbodorsal aponeurosis common to it and the internal oblique. The cranial fibres pass nearly ventrad, the caudal fibres caudad, and the intervening fibres take an intermediate course. The fibres end in a thin aponeurosis of insertion along a curved line which passes at first caudad and then laterodorsad. The aponeurosis fibres continue in the direction of the muscle-fibres to the