Part 36

Insertion on the first rib and the lateral portion of its costal cartilage.

Relations.--Outer surface with the pectoralis minor (Fig. 65, o). Inner surface with the scalenus (Fig. 73, f‴), the rectus abdominis (Fig. 73, k), and the intercostales externi (Fig. 73, m). At its insertion it is united with a portion of the scalenus (Fig. 73, f‴).

Action.--Draws the sternum forward.

=Mm. levatores costarum.=--Small muscles having origin on the transverse processes of the thoracic vertebræ, passing caudoventrad, and becoming inserted on the angle of the rib lying immediately caudad of the origin. They are continuous with the external intercostals.

Relations.--Outer surface with the longissimus dorsi (Fig. 69, f), and craniad with the iliocostal (Fig. 69, h). Inner surface with the internal intercostals (Fig. 69, k).

Action.--Pull the ribs dorsocraniad.

=Mm. intercostales externi= (Fig. 73, m; Fig. 69, i).--The external intercostals are placed in the outer portion of the intercostal spaces. They are composed of bundles of fibres attached by their ends to the adjacent borders of the ribs and having in general the direction of the external oblique muscle, i.e., they pass from their cranial ends caudoventrad. They occupy the intercostal spaces between the true ribs and extend even caudad into the spaces between the false ribs. They are lacking between the ventral ends of the costal cartilages of the first six to eight ribs, so that the internal intercostals (Fig. 69, k) are here exposed. The more caudal external intercostals are more nearly craniocaudal in direction.

Relations.--Outer surface with obliquus abdominis externus (Fig. 68, p), latissimus dorsi (Fig. 68, m), serratus posterior inferior (Fig. 73, n) and superior (Fig. 73, l), serratus anterior (Fig. 73, i), scalenus (Fig. 73, f), and iliocostal (Fig. 69, h). Inner surface with the internal intercostals (Fig. 69, k).

Action.--Protractors of the ribs.

=Mm. intercostales interni= (Fig. 69, k).--The internal intercostals are similar to the external intercostals, beneath which they lie. Their fibres pass between the ribs at nearly right angles to those of the external intercostals and have nearly the direction of the fibres of the internal oblique. They occupy all the intercostal spaces from the first to the thirteenth ribs.

Relations.--Outer surface with the external intercostals (Fig. 69, i), and ventrad with the scalenus (Fig. 73, f), transversus costarum (Fig. 73, j), and rectus abdominis (Fig. 73, k). Inner surface with the pleura and the transversus thoracis.

Action.--Retractors of the ribs.

=M. transversus thoracis= (triangularis sterni; sternocostalis internus).--This represents a thoracic portion of the transversus abdominis. It consists of five or six flat muscular bands lying on the inner surface of the thoracic wall.

Origin on the lateral borders of the dorsal face of the sternum, opposite the attachments of the cartilages of the third to the eighth ribs. The six bands thus formed, each about one centimeter wide, pass laterad and are inserted into the cartilages of the ribs near their junction with the ribs, and into the fascia which covers the inner surface of the internal intercostals in this region.

Relations.--Outer surface with the internal intercostals and the cartilages of the ribs. Inner surface with the pleura.

=Diaphragma= (Fig. 74).--The diaphragm consists of a central so-called =semilunar= tendon (e) and of muscular fibres which pass radially from the body wall to the tendon. It forms a complete oblique partition between the abdominal and thoracic cavities. The dorsal end is farther caudad than the ventral.

The central tendon (e) is thin and irregularly crescent-shaped, with the convexity ventrad and the horns of the crescent prolonged as two tendinous bands (e′) which end in two triangular =membranous= portions (d) of the diaphragm, one on each side of the spinal column. It is pierced by an opening for the vena cava (f).

a, right crus; b, left crus; c, c′, sternocostal part of diaphragm; d, d′, membranous portions of the diaphragm; e, central tendon; e′, prolongations of central tendon; f, opening for posterior vena cava; g, œsophagus; h, aorta; i, M. transversus abdominis.]

The muscular portion is in two parts: (a) The =vertebral= portion (a, b) arises by a single tendon from the ventral surface of the second, third, and fourth lumbar centra. The tendon diverges into two, the right one (a) of which is much stronger, and from each of the two arise muscle-fibres. Each mass of fibres is one of the two =crura= (sing. =crus=) of the diaphragm. The aorta (h) enters the abdomen between the crura dorsally. The fibres of each crus diverge to be inserted into the central tendon and the dorsal continuation of its horn. The two sets of fibres unite ventrad of the opening of the aorta. Between this opening and the central tendon is another opening for the œsophagus (g); this lies entirely in that part of the diaphragm which arises from the right crus (a). The fibres are again united ventrad of the œsophageal opening.

(b) The =sternocostal part= (c, c′) takes origin from the xiphoid process and the last five ribs, by fleshy bundles which interdigitate with those of the transversus abdominis (i). The fibres converge to the central tendon (e). Between the crus of each side and the most dorsal of the costal fibres is the membranous interval (d) mentioned above.

4. =Abdominal Muscles.= =M. obliquus abdominis externus= (Fig. 68, p).--A large, thin sheet of muscle covering the whole abdomen and part of the thorax ventrally.

Origin.--(a) From the last nine or ten ribs by means of as many tendons, which are interconnected to form arches that span the slips of the serratus anterior. The muscle-fibres arise from these tendons and from their intervening arches. (b) From the lumbodorsal aponeurosis common to it and the internal oblique. The cranial fibres pass nearly ventrad, the caudal fibres caudad, and the intervening fibres take an intermediate course. The fibres end in a thin aponeurosis of insertion along a curved line which passes at first caudad and then laterodorsad. The aponeurosis fibres continue in the direction of the muscle-fibres to the

Insertion into the median raphe ventrad of the sternum from the insertion of the seventh costal cartilage to the xiphoid process, into the linea alba from the sternum to the pubic tubercle, and into the tubercle and the cranial border of the pubis. Caudad of the xiphoid process the aponeurosis is closely united to the superficial layer of the internal oblique, where it forms the outer layer of the sheath of the rectus abdominis. Laterad of the pubic tubercle the tendon is perforated by the inguinal canal. In the cat neither the caudal part of the muscle nor its tendon is attached to the ilium, as it is in man and the dog, so that no Poupart’s ligament, or inguinal ligament, is formed.

Relations.--Outer surface with the cutaneus maximus, the integument, and near the origin with the latissimus dorsi (Fig. 68, m). Inner surface with the obliquus internus (Fig. 73, o), the rectus abdominis (Fig. 73, k), the intercostales externi (Fig. 73, m), a small part of the serratus posterior inferior (Fig. 73, n), and by its dorsal tendon with the longissimus dorsi (Fig. 69, f).

Action.--Constrictor of the abdomen.

=M. obliquus abdominis internus= (Fig. 73, o).--A thin sheet similar to the preceding but of less extent. Its fibres cross those of the external oblique nearly at right angles and lie beneath them.

Origin.--(1) Between the fourth and seventh lumbar vertebræ from the lumbar aponeurosis which is common to it and the external oblique. The lumbar aponeurosis takes origin from the lumbar spinous processes and the interspinous ligaments, and is continuous craniad with the aponeurosis of the serratus posterior inferior. Laterad it splits into three sheets, two for the above-named muscles and a third which passes to the vertebral transverse processes and forms the fascia covering the supraspinous muscles of the lumbar region. (2) By a similar aponeurosis from the ventral half of the iliac crest. (3) By fleshy fibres from the three =crural arches=. These are three delicate ligamentous arches which stretch from the crest of the ilium to the pubic spine. The dorsal one gives exit to the iliopsoas muscle, the middle to the femoral vessels, and the ventral to the spermatic cord. In the female the middle and ventral arches may fuse. The pillar between the dorsal and middle arches is attached to the iliopectineal eminence.

Insertion.--The fibres pass cranioventrad and end along a longitudinal line in a thin aponeurosis of insertion, which is united in the linea alba to those of the external oblique and transversus. At the caudal end of the aponeurosis all its fibres pass outside of the rectus abdominis (Fig. 73, k). At its cranial end the fibres divide into two sheets or laminæ, one of which passes outside of the rectus and the other inside. The outer lamina unites with the aponeurosis of the external oblique, while the inner lamina unites with that of the transversus. There is thus formed a sheath for the cranial part of the rectus muscle.