432 THE MUSCULATURE 



the thorax. Together with the serratus posterior superior and the connecting aponeurotic 

 fascia it aids in keeping the intrinsic dorsal muscles in place. 



Relations. — It lies upon the intrinsic dorsal musculature, the lower dorsal part of the 

 thorax, and the lumbo-dorsal fascia, and beneath the latissimus dorsi, the trapezius, and their 

 aponeuroses. 



Variations. — The fasciculi of both muscles vary in number and may be replaced by ap- 

 oneurotic slips. Aberrant muscle fasciculi, supracostales posteriores, may be found in the fascia 

 which connects the two muscles. In several of the lower mammals the two muscles are nor- 

 mally continuous. 



2. External Oblique Group 



The intercostales externi (fig. 385). — These muscles extend in the intercostal spaces from 

 the tubercles of the ribs to the costal cartilagess. The intermediate muscles do not, however, 

 often quite reach the cartilages. The first intercostal muscle may extend to the sternum. The 

 others are continued through the intercostal region by thin aponeuroses, the external inter- 

 costal ligaments, the fibres of which have a direction corresponding to that of the muscle fibre- 

 bundles. Dorsally the muscles are fused with the levatores, and ventrally the lower seven 

 muscles are more or less fused with the corresponding fasciculi of the external oblique. 



Origin. — -From the lower margin of each rib external to the costal sulcus. 



Structure and insertion. — The fibre-bundles take a parallel course obliquely forward and 

 downward to the upper margin of the next rib. The proximal fibre-bundles are more oblique 

 than the distal, and the muscles are best developed in the dorsal part of the intercostal spaces. 



Nerve-supply. — By several branches from the corresponding intercostal nerves. 



Action.^To elevate the ribs and enlarge the thorax. 



Relations. — They are covered externally by the pectoral muscles, the serratus anterior, 

 and serrati posteriores, the levatores costarum, the sacro-spinalis (erector spinse), and the 

 external oblique muscles. Internally they are separated by a slight amount of loose tissue from 

 the internal intercostals, the membranes which continue these muscles medially, and from the 

 subcostal muscles. 



Variations. — When the twelfth'^rib is very small or is lacking, the eleventh intercostal 

 muscle may be missing. When there is a supernumerary cervical or thirteenth thoracic rib, 

 there may be an extra external intercostal muscle. Next to the first intercostal, the fourth 

 most frequently reaches the sternum. 



The levatores costarum (fig. 383). — These consist of a series of flat, triangular muscles, 

 each of which arises from the tip and inferior margin of a transverse process and extends later- 

 ally with diverging fibre-bundles to be inserted into the dorsal surface of the rib below, from 

 the tubercle to the angle. The first extends from the transverse process of the seventh cervical 

 vertebra to the first rib. They increase successively in size from this to the last, which is at- 

 tached to the twelfth rib. Those arising from the transverse processes of the eighth to the elev- 

 enth thoracic vertebra; send their more medial fibre-bundles across the rib below to join the 

 lateral margin of the succeeding muscle (levatores longi). The levatores costarum are closely 

 united to the external intercostals and are innervated by the intercostal nerves which pass for- 

 ward in the corresponding intercostal spaces. The first muscle is innervated by the eighth 

 cervical nerve. 



Action. — To bend laterally and extend the spinal column. 



Relations. — They are covered dorsally by the longissimus dorsi and the ilio-costalis. 



Variations. — The first levator may be continued into the scalenus posterior. When greatly 

 developed, the series of levators forms a serrate muscle. 



The obliquus abdominis externus (fig. 387). — Oriqin. — By eight fleshy digitations from 

 the external surface of the lower eight ribs immediately lateral to where they join the cartilages. 

 The first five slips interdigitate with the serratus anterior (magnus), the last three with the 

 lati.ssimus dorsi. 



Insertion. — (1) By a strong aponeurosis which extends over the rectus to the linca alba, 

 where the more superficial fibres interdigitate across the median line, and to the inguinal (Pou- 

 part's) ligament; and (2j directly into tlie outer lip of the crest of the ilium. The aponeurosis 

 over the rectus is usually partly fused with the aponeurosis of the internal oblique. 



Structure. — The fibre-l)uiidles which compose the flat fasciculi of origin diverge slightly 

 as they pass forward and ilownward, and by fusion of their edges give ri.se to a flat sheet of 

 muscle. The fasciculus taking origin from the fifth rib passes nearly directly ventrally, but 

 the succeeding fasciculi incline somewhat downward, those from the seventh to the ninth ribs 

 showing the greatest downward inclination. The lower margin of the fasciculus which arises from 

 the seventh rib terminates opposite the umbilicus, that from the ninth rib extends toward the 

 anterior superior spine of the ilium, and those from the last three ribs descend to the iliac crest. 

 The first two fasciculi extend over the lateral margin of the rectus, the next two to its lateral 

 edge. The fourMi and fifth usually terminate along a line extending ventrally from the anterior 

 superior iliac spine toward tiie rectus. 



Nerve-sup ply. -~V\w external oblique is supplied by rami from the lateral branches of the 

 lower .seven intercostal iierv((s and usually from the ilio-hypogastric as well. The rami of the 

 first two or three nerves usually extend on the external surfac^e of the muscle, while tlie others 

 extend on the deep surface of the muscle as the cutaneous l)ranches are passing through it to- 

 ward the skin. The nerves of the external oblique take a more transverse direction than the 

 fasciculi of the muscle. Thus the branch from the tenth intercostal nerve extends toward the 

 umbilicus and that of the twelfth toward a point midway between the uiii})ilicus and the sym- 

 physis pubis. The nerves have a segmental distribution (corresponding with the primitive 

 segmental condition of the muscle. 



Action. — (1) To compress the abdomen; (2) to (lei)ress the thorax; (3) to flex the spinal 



