434 THE MUSCULATURE 



costal muscles, the diaphragm, and the pleural membranes. The more distal internal inter- 

 costal muscles are continuous with the internal oblique and the subcostal muscles. 



Variations. — The tenth and eleventh internal intercostal muscles normally are but slightly 

 developed and often may be wanting. The internal intercostals of the first three spaces may 

 extend to the vertebrae. 



The subcostales (fig. 385). — These muscles are due to an extension over two or more inter- 

 costal spaces of those fibre-bundles of the internal intercostal muscles which lie in the proximal 

 part of the interspaces. They arise near the angles of the ribs, and are usually well developed 

 only in the lower part of the thorax. The component fibre-bundles keep the general direction 

 of the internal intercostals, but they converge toward the tendons of insertion, which are at- 

 tached in each case to the second or third rib below, between the angle and the neck. 



Nerve-supply. — The main nerve of supply for each muscle comes from the intercostal nerve 

 running below the rib from which the muscle takes origin. 



Action. — To depress the ribs and contract the thorax. 



Relations. — They lie on the inner side of the internal and external intercostals and the ribs, 

 and are covered by "the pleural membranes. 



Variations. — They vary much in development. Next to the lower fasciculi, the fasciculi 

 in the cranial part of the thorax are those usually best developed. 



The obliquus abdominis internus (fig. 388). — Origin.— From the lumbo-dorsal fascia the 

 intermediate hp of the ventral two-thirds of the iliac crest, and the lateral half of the inguinal 

 ligament. 



Structure and insertion. — From the origin the fibre-bundles radiate forward in a flat sheet. 

 The most dorsal extend to the lower three ribs, where they become continuous with the internal 

 intercostals. The rest extend toward the lateral margin of the rectus, the upper ones toward 

 the xiphoid process, the intermediate toward the umbilicus, the lower ones somewhat obliquely 

 downward across the lower part of the abdomen. The fibre-bundles which extend toward the 

 rectus terminate in an aponeurosis which in its upper two-thirds divides into two layers, one 

 of which passes in front of and the other behind the rectus muscle to the linea alba. In the 

 lower third the aponeurosis passes as a single membrane in front of the rectus. In the neigh- 

 bourhood of the subcutaneous inguinal (external abdominal) ring the muscle is continued into 

 the cremaster. Medial to the ring some fasciculi are attached to the tubercle of the pubis 

 and to the sj^mphysis. 



Nerve-supphj. — From branches of the last three intercostal and the ilio-hypogastric, ilio- 

 inguinal and genito-femoral (?) nerves as these pass between this muscle and the transversus. 



Action. — To depress the thorax, flex the vertebral column, and bend and rotate it toward 

 the side on which the muscle is placed. When the thorax is fixed, the muscle serves to flex and 

 rotate the pelvis. 



Relations. — It lies between the external oblique and the transversus. The trigonum lum- 

 bale (triangle of Petit) is an area, variable in size, between the posterior margin of the external 

 oblique, the lateral margin of the latissimus dorsi, and the crest of the ilium. In this area the 

 internal obhque is subcutaneous. 



Variations. — The attachments and the extent of development of the fleshy part of the 

 muscle vary considerably. Occasionally tendinous inscriptions are found in the muscle which 

 indicate a primitive segmental condition. 



The cremaster (fig. 389). — The cremaster muscle is found well developed only in the male. 

 It represents an extension of the lower border of the internal oblique muscle and possibly a!so 

 of the transverse over the testis and spermatic cord. 



Origin. — (1) Lateral, thick and fleshy, from about the middle of the upper border of the 

 inguinal ligament, and (2) medial, thin and tendinous, from the sheath of the rectus muscle and 

 the tubercle (spine) of the pubis. 



Structure.— The lateral head is applied to the lateral side, the medial head to the medial side, 

 of the spermatic cord. Both pass with this through the subcutaneous (external abdominal) 

 ring of the inguinal canal and become spread in loops over the testis. Ensheathing the muscle 

 and between the somewhat scattered fibre-bundles which compose it, there extends a thin, 

 membranous layer of connective tissue, the cremasteric (Cowper's) fascia. 



Nerve-supply. — The genital nerve (external spermatic), usually joined by a ramus from the 

 inguinal nerve, gives rise to branches which sjjread over the muscle. 



Action. — To lift the testis toward the subcutaneous inguinal (external abdominal) ring. 



Relations. — It is covered by the aponeurosis of the external oblique, the cremasteric fascia, 

 the dartos, and the skin. It covers the spermatic cord and the testis. 



Variations. — In the female the muscle is represented by a few fasciculi on the round liga- 

 ment. It may arise wholly from the transversalis fascia or be somewhat fused with the trans- 

 versus muscle. The latter condition is especially frequent in muscular individuals. 



4. Transversus Group 



The transversus thoracis (triangularis stcrni) (fig. 386). — Origin.— Jiy aponeurotic bands 

 from the dorsal surface of the lower lialf of the l)ody of the sternum and the xi])hoi(i process. 



Structure and insertion. — -The muscle is composed of several flat, thin fas(^i(Hili, partly 

 fibrous, more or less isolated, which are inserted by aponeurotic bands into the dorsal surface 

 of the cartilages of the second or third to the sixth ribs, and occasionally also into the tips of 

 the bony portions of the ribs. The lower fasciculus is closely related to the cranial margin of 

 the transversus abdominis. 



Nerve-supply. — Hy rami from the ventral portions of the second to the sixth intercostal 

 nerves. These nerves give rise to a longitudinal plexus across the deep surface of the muscle 

 near the middle of the constituent fasciculi. 



