T. The left common iliac vein, joining the right under the right common iliac artery to form the inferior vena cava. U. Fifth lumbar vertebra. V. The external iliac artery of right side. W. The symphysis pubis. X. An incision made over the locality of the femoral artery. b b. The dorsal intercostal arteries. c. The coeliac axis d. The superior mesenteric artery. f f. The renal arteries. g. The inferior mesenteric artery. h. The vas deferens bending over the epigastric artery and the os pubis, after having passed through the internal abdominal ring. [Illustration: Chest and abdomen, showing bones, blood vessels, muscles and other internal organs.] Plate 25 COMMENTARY ON PLATE 26. THE RELATION OF THE INTERNAL PARTS TO THE EXTERNAL SURFACE OF THE BODY. An exact acquaintance with the normal character of the external form, its natural prominences and depressions, produced by the projecting swell of muscles and points of bone, &c., is of great practical importance to the surgeon. These several marks described on the superficies he takes as certain guides to the precise locality and relations of the more deeply situated organs. And as, by dissection, Nature reveals to him the fact that she holds constant to these relations, so, at least, may all that department of practice which he bases upon this anatomical certainty be accounted as rooted in truth and governed by fixed principles. The same organ bears the same special and general relations in all bodies, not only of the human, but of all other species of vertebrata; and from this evidence we conclude that the same marks on surface indicate the exact situation of the same organs in all similar bodies. The surface of the well-formed human body presents to our observation certain standard characters with which we compare all its abnormal conditions. Every region of the body exhibits fixed character proper to its surface. The neck, the axilla, the thorax, the abdomen, the groin, have each their special marks, by which we know them; and the eye, well versed in the characters proper to the healthy state of each, will soonest discover the nature of all effects of injury--such as dislocations, fractures, tumours of various kinds, &c. By our acquaintance with the perfect, we discover the imperfect; by a comparison with the geometrically true rectangled triangle, or circle, we estimate the error of these forms when they have become distorted; and in the same way, by a knowledge of what is the healthy normal standard of human form, we diagnose correctly its slightest degree of deformity, produced by any cause whatever, whether by sudden accident, or slowly-approaching disease.