416 DISSECTION OF THE ABDOMEN. 



in number, and have the following position : one is opposite the umbilicus, 

 another at the ensiform cartilage, and the third midway between the two. 

 If there is a fourth, it will be placed below the umbilicus. These mark- 

 ings seldom extend the whole length or breadth of the muscular fibres, 

 more particularly above and below. 



Linea semilunaris (fig. 139). This line corresponds with the outer 

 edge of the rectus, and reaches from the eighth rib to the pubic spine of the 

 hip-bone : it marks the line of division of the aponeurosis of the internal 

 oblique muscle. 



The PYRAMIDALIS MUSCLE (fig. 139) is triangular in form, and is placed 

 in front of the rectus near the pelvis. The muscle arises by its base from 

 the front of the pubes, and is inserted into the linea alba about midway 

 between the umbilicus and the pelvis. This small vessel is often absent. 



Action. The muscle renders tight the linea alba ; and when large it 

 may assist the rectus slightly in compressing the viscera. 



NERVES OF THE ABDOMINAL WALL (fig. 138). Between the internal 

 oblique and transversalis muscles are situate the intercostal nerves ; and 

 near the pelves are two branches of the lumbar plexus. Some arteries 

 accompany the nerves, but they will be referred to with the vessels of the 

 abdominal wall. 



The lower six intercostal nerves (fig. 138ttt) enter the wall of the 

 abdomen from the intercostal spaces. Placed between the two deepest 

 lateral muscles, the nerves are directed forwards to the edge of the rectus, 

 and through this muscle to the surface of the abdomen near the middle 

 line. About midway between the spine and the linea alba, the nerves 

 furnish cutaneous branches to the side of the abdomen (lateral cutaneous, 

 p. 406) ; and whilst between the abdominal muscles they supply muscular 

 branches, and offsets of communication with one another. A greater part 

 of the lower than of the upper nerves is visible, owing to the shortness of 

 the last intercostal spaces. 



The last dorsal nerve (*) is placed below the twelfth rib, and therefore 

 not in an intercostal space, but it has connections and a distribution like 

 the preceding. As it extends forwards to the rectus it communicates 

 sometimes with the ilio-hypogastric nerve. Its lateral cutaneous branch 

 perforates the two oblique muscles (p. 406). 



Two branches of the lumbar plexus, viz., ilio-hypogastric and ilio-ingui- 

 nal, are contained for a certain distance between the muscles of the wall of 

 the abdomen, as they course forwards to the surface of the body. 



The ilio-hypogastric nerve ( 2 ) perforates the back of the transversalis 

 muscle near the iliac crest, and gives off the iliac branch. The nerve is 

 then directed forwards above the hip bone, and is connected with its com- 

 panion (ilio-inguinal). Perforating the fleshy part of the internal oblique 

 near the front of the iliac crest, and the aponeurosis of the external oblique 

 near the linea alba, the nerve becomes cutaneous (p. 406). 



Its iliac branch pierces both oblique muscles close to the crista ilii, to 

 reach the gluteal region. 



The ilio-inguinal nerve perforates the transversalis muscle near the 

 front of the iliac crest. It pierces afterwards the internal oblique, and 

 reaches the surface of the thigh through the external abdominal ring (p. 

 407) ; it furnishes offsets to the internal oblique, the cremaster, and the 

 pyramid alls. 



Dissection. For the purpose of seeing the transversalis fascia, it will 

 be necessary to raise, on the right side, the lower part of the transversalis 



