SERRATUS MUSCLES 431 



bilicus. Another, corresponding to the ninth rib, is situated midway between this and the lower 

 margin of the thoracic wall, and one corresponding to the seventh rib is found at the level of the 

 xiphoid process. Between this and the one corresponding to the ninth rib an additional inscrip- 

 tion is frequently found. Below the umbilicus an inscription corresponding with the eleventh 

 rib is often found (30 per cent.). In these inscriptions many of the fibre-bundles have their 

 origin and insertion. The thoracic attachments take place by means of band-like fasciculi 

 which extend upward from the highest inscription, the fibre-bundles of these fasciculi being 

 inserted by short tendinous bands. The pubic attachment of the muscle takes place by a 

 short, thick tendon, usually divisible into two portions, of which the broader, lateral portion is 

 inserted into a rough area extending from the pubic tubercle (spine) to the symphysis, while the 

 more slender medial portion is attached to the fasciae in front of the symphysis pubis, where its 

 jSbres interdigitate with those of the opposite side. In addition to the attachments mentioned, 

 some of the fibre-bundles are attached to the sheath of the rectus and many, after interdigitating, 

 terminate in the intramuscular framework. 



Nerve-supply. — The anterior branches of the six or seven lowermost intercostal nerves 

 enter the deep surface of the muscle near its lateral edge. The cutaneous branches pass 

 obliquely through its substance, while the muscular branches give rise to an intramuscular 

 plexus. As a rule, the chief ventral branch of the tenth thoracic nerve enters the substance 

 of the muscle slightly below the umbilical transverse inscription. The branches of the eleventh 

 and twelfth nerves enter at a lower level. The main branch of the ninth nerve enters slightly 

 below the preumbilical inscription; the eighth nerve, between this and the lower margin of 

 the thorax. Either the sixth or seventh nerve may supply the fasciculi of origin. In addition 

 to the main branches other smaller branches of the nerves of the abdominal wall are also usually 

 distributed to the muscle. Each segment, either directly or through intramuscular plexuses, 

 has a supply from more than one spinal nerve. 



Action. — To depress the thorax and flex the spinal column. When the thorax is fixed the 

 rectus serves to flex the pelvis upon the trunk. 



Relations. — It lies between the transversalis fascia and the tela subcutanea and is ensheathed 

 by the aponeuroses of the lateral abdominal muscles, as above described. The epigastric artery 

 runs on its deep surface. 



Variations. — The rectus muscle varies in the number of its tendinous inscriptions and in 

 the extent of its thoracic attachment. It may extend farther than usual on the thorax. Fre- 

 quently aponeurotic slips or slips of muscle on the upper part of the thorax indicate a more 

 primitive condition in which the muscle extended to the neck. Absence of a part or the whole 

 of the muscle has been noted. The muscles of the two sides may be separated by a considerable 

 interval in the neighbourhood of the umbilicus. The muscle is relatively thicker in men than 

 in women. 



The pyramidalis (fig. 388). — Origin. — Upper border of the body of the pubis. 



Structure and insertion.— The fibre-bundles extend toward and are inserted into the linea 

 alba for about a third of the distance to the umbilicus, and give rise to a flat, triangular belly. 



Nerve-supply. — Usually through a branch of the twelfth thoracic, which may extend into 

 the muscle through the rectus abdominis. Not infrequently a special branch extends into the 

 muscle from the ilio-hypogastric or ilio-inguinal or, rarely, from the genito-femoral. 



Action. — To draw down the linea alba in the median line. 



Relations. — It lies between two laminae of the anterior layer of the sheath of the rectus. 



Variations.— It is missing in about 16 per cent, of instances (Le Double). Dwight has 

 found it absent in 81 out of 450 males and in 60 out of 223 females dissected at the Harvard 

 Medical School. It may extend upward to the umbilicus or be but very slightly developed. 

 It may be double. In many of the mammals it is missing. It is well developed in the marsu- 

 pials and monotremes. 



B. Lateral Division 



1, Serratus Group (fig. 380) 



The serratus posterior superior.^Ori^m. — By a broad, thin aponeurosis from the Uga- 

 mentum nuchae and the spines of the last one or two cervical and the first two or three thoracic 

 vertebrae. 



Structure and insertion. — The fibre-bundles take a nearly parallel course downward and 

 lateralward and give rise to a flat belly which ends by four fasciculi on the upper margin of 

 the second to the fifth ribs, lateral to the ilio-costahs. 



Nerve-supply. — Through branches from the first four intercostal nerves. These nerves 

 give rise to a plexus which passes across the deep surface of the muscle in the middle third be- 

 tween the tendons of origin and insertion. 



Action. — ^To elevate the ribs to which the muscle is attached, and through them to enlarge 

 the thorax. 



Relations. — It lies upon the wall of the thorax and the intrinsic dorsal musculature and 

 beneath the levator scapulae, rhomboids, serratus anterior, and trapezius. Its fascicuH extend 

 on the ribs to those of the serratus anterior (magnus). 



The serratus posterior inferior. — Origin. — Through an aponeurosis, fused medially and 

 inferiorly with the lumbo-dorsal fascia, from the last two or three thoracic and first two or three 

 lumbar spines. 



Structure and insertion. — From the aponeurosis arise four flat bands which are successively 

 attached to the inferior margins of the last four ribs, lateral to the ilio-costalis. 



Nerve-supply. — From the ninth to eleventh intercostal nerves arise branches which form a 

 plexus that extends across the deep surface of the muscle in the middle third between the ten- 

 dons of origin and insertion. 



Action. — To depress and draw outward the four lower ribs and through them to enlarge 



