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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
 
7. Surface Anatomy of the Abdomen
 

Skin.—The skin of the front of the abdomen is thin. In the male it is often thickly hair-clad, especially toward the lower part of the middle line; in the female the hairs are confined to the pubes. Just below the line of the iliac crest, especially marked in fat subjects, is a shallow groove termed the iliac furrow, while in the site of the inguinal ligament a sharper fold known as the fold of the groin is easily distinguishable.
  After distension of the abdomen from pregnancy or other causes the skin commonly presents transverse white lines which are quite smooth, being destitute of papillæ; these are known as striæ gravidarum or striæ albicantes. The linea nigra of pregnancy is often seen as a pigmented brown streak in the middle line between the umbilicus and symphysis pubis.
  In the middle line of the front of the abdomen is a shallow furrow which extends from the junction between the body of the sternum with the xiphoid process to a short distance below the umbilicus; it corresponds to the linea alba. The umbilicus is situated in the middle line, but it varies in position as regards its height; in an adult subject it is always placed above the middle point of the body, and in a normal well-nourished subject is from 2 to 2.5 cm. above the level of the tubercles of the iliac crests.

Bones.—The bones in relation with the surface of the abdomen are (1) the lower part of the vertebral column and the lower ribs and (2) the pelvis; the former have already been described (page 1303), the latter will be considered with the lower limb.

Muscles (Fig. 1219).—The only muscles of the abdomen which have any considerable influence on surface form are the Obliquus externus and the Rectus. The upper digitations of origin of Obliquus externus are well-marked in a muscular subject, interdigitating with those of Serratus anterior; the lower digitations are covered by the border of Latissimus dorsi and are not visible. The attachment of the Obliqui externus and internus to the crest of the ilium forms a thick oblique roll which determines the iliac furrow. Sometimes on the front of the lateral region of the abdomen an undulating line marks the passing of the muscular fibers of the Obliquus externus into its aponeurosis. The lateral margin of the Obliquus externus is separated from that of the Latissimus dorsi by a small triangular interval—the lumbar triangle—the base of which is formed by the iliac crest, and its floor by Obliquus internus.
  The lateral margin of Rectus abdominis is indicated by the linea semilunaris, which may be exactly defined by putting the muscle into action. The surface of the Rectus presents three transverse furrows, the tendinous inscriptions: the upper two of these, viz., one opposite, or a little below, the tip of the xiphoid process, and the other midway between this point and the umbilicus, are usually well-marked; the third, opposite the umbilicus, is not so distinct. Between the two Recti the linea alba can be palpated from the xiphoid process to a point just below the umbilicus; it is represented by a distinct dip between the muscles: beyond this the muscles are in apposition.

Vessels.—In thin subjects the pulsation of the abdominal aorta can be readily felt by making deep pressure in the middle line above the umbilicus.

Viscera.—Under normal conditions the various portions of the digestive tube cannot be identified by simple palpation. Peristalsis of the coils of small intestine can be observed in some persons with extremely thin abdominal walls when some degree of constipation exists. In cases of constipation it is sometimes possible to trace portions of the great intestine by feeling the fecal masses within the gut. In thin persons with relaxed abdominal walls the iliac colon can be felt in the left

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