Part 32

=M. extensor caudæ lateralis= (Fig. 70, f).--This is a continuation caudad of the medial portion of the longissimus dorsi (Fig. 70, a); it lies just laterad of the extensor caudæ medialis (Fig. 70, e).

Origin in many fleshy bundles from the articular processes of the sacral vertebræ, and the transverse processes of the caudal vertebræ. The fibres curve dorsocaudad and are inserted by many long slender tendons on the dorsal surfaces of the caudal vertebræ. The muscle grows continually smaller as it passes caudad.

Action.--Raises the tail.

=M. abductor caudæ= (coccygis) =externus= (Fig. 68, w; Fig. 70, g).--A rounded muscle on the lateral surface of the cranial part of the tail.

Origin from the medial side of the dorsal border of the ilium, and from the dorsal surface of the sacrum. The muscle passes caudad, lying just ventrad of the extensor lateralis, and is inserted into the transverse processes and lateral surfaces of the caudal vertebræ, as far back as the eighth or ninth.

Action.--Bends the tail sideways.

=M. abductor caudæ internus= (or M. coccygeus).--A large flat muscle, having origin on the spine of the ischium. The muscle passes dorsomediad, spreading out, and is inserted into the transverse processes of the second to the fourth caudal vertebræ.

Relations.--Lateral surface with the caudofemoralis (Fig. 68, s), gluteus maximus, and pyriformis. Medial surface with the iliocaudalis and the flexor caudæ longus (Fig. 68, x).

=M. iliocaudalis= (Str.-D.) (Fig. 162, 11′, page 398).--This represents a portion of the levator ani of man, and in some specimens is united with that muscle (Fig. 162, 11) in the cat.

Origin along the ventral half of the medial surface of the ilium, caudad of the sacrum. The fibres pass caudad and are inserted by a flat tendon into the ventral surface of the caudal vertebræ, from the second or third to about the seventh.

Relations.--Lateral surface with the gluteus maximus and abductor caudæ internus (Fig. 68, w). Medial surface with the levator ani (Fig. 162, 11), with which this muscle may be partly united.

Action.--The two muscles together flex the tail.

=M. flexor caudæ longus= (Fig. 162, 12; Fig. 113, c, page 270).

Origin on the ventral surface of the last lumbar vertebra, of the sacrum, and of the transverse processes of the caudal vertebræ. Caudad the muscle forms long, strong tendons which are inserted on the ventral surface of the tail.

Relations.--Lateral surface with the ilium, the gluteus maximus, the abductor caudæ internus, the iliocaudalis, and the skin of the tail. Dorsally it touches in the caudal region the abductor caudæ externus (Fig. 68, w), medially the flexor caudæ brevis (Fig. 162, 13).

Action.--Flexes the tail.

=M. flexor caudæ brevis= (Fig. 162, 13).

Origin on the ventral surface of the caudal vertebræ, from the first to the seventh or eighth. The principal head comes from the first vertebra. The bundles thus formed pass caudad and are inserted each into the ventral surface of a vertebra some distance caudad of the origin. The muscle extends to about the tenth caudal vertebra, and its most caudal parts are closely interconnected with the flexor caudæ longus.

Relations.--Lateral edge with the flexor caudæ longus. Medial edge in the sacral region with the levator ani (Fig. 162, 11) and the iliocaudalis (Fig. 162, 11′); in the caudal region with the muscle of the opposite side.

Action.--Flexes the tail.

2. =Muscles on the Ventral Side of the Vertebral Column.=

A. LUMBAR AND THORACIC REGIONS.--The iliopsoas (Fig. 162, 8) belongs in this group, but since it moves the leg it is described with the muscles of the pelvic limbs.

=M. psoas minor= (Fig. 162, 9, page 398).--This muscle lies along the ventral surface of the vertebral column, next to the peritoneum, and extends from the thoracic vertebræ to the ilium.

Origin usually by five heads, from the caudal border of the centra of the last two (or one) thoracic and first three (or four) lumbar vertebræ. These heads also serve as origin for a part of the iliopsoas, and are closely united at their origin with the quadratus lumborum. They unite to form an, at first, rather large flat muscle, which rapidly becomes smaller caudad, finally forming a slender tendon which is inserted on the iliopectineal line, just craniad of the acetabulum.

Relations.--Dorsolateral surface with the quadratus lumborum and iliopsoas (Fig. 162, 8). Ventral surface with the pleura and diaphragm near the origin; with the peritoneum caudad.

Action.--Flexes the back in the lumbar region.

=M. quadratus lumborum.=--A flat muscle lying on the ventral surface of the transverse processes of the lumbar vertebræ. Craniad it has origin on the ventral surface of the last two thoracic vertebræ, and by a few fibres from the last rib. The muscle passes caudad, lying on the lateral side of the psoas minor (Fig. 162, 9), and becomes attached to each of the transverse processes of the lumbar vertebræ. Caudad the muscle passes into a strong flat tendon which is inserted into the anterior inferior spine of the ilium.

Relations.--Dorsal surface with the transverse processes and the intertransverse muscles. Ventral and medial surfaces with the psoas minor (Fig. 162, 9) and iliopsoas (Fig. 162, 8).