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DEEP ABDOMINAL REGION. 
THE ABDOMEN IS BOUNDED by a muscular wall anteriorly, which forms 
a movable septum between it and the chest, and is called the diaphragm. 
Superiorly also there are the muscles which serve to bend the spine 
downwards, in opposition to the dorsal muscles. 
THE DIAPHRAGM consists of a large flat muscle and two crura, with 
a thin circular layer of tendon in the centre. The former arises by fleshy 
digitations from the cartilages of the ribs, from the eighth to the 
sixteenth inclusive, and from the ensiform cartilage. It is inserted into 
a central flat tendon of a circular shape. Lach crus arises from its cor- 
responding side of the bodies of the lumbar vertebrae: the two cross each 
other opposite the seventeenth dorsal vertebra, and again decussate after 
allowing the esophagus to pass through, being finally attached to the 
central tendon. Between the crura and the bodies of the vertebra the 
aorta passes backwards, and in the central tendon is the opening for the 
vena cava posterior. The action of this muscle is to diminish the capacity 
of the thorax by reducing the convexity of its surface. 
SEMI-SPINALIS LUMBORUM, INTERTRANSVERSALIS LUMBORUM, and SACRO 
LUMBALIS, are three muscles having numerous attachments to the trans- 
verse processes of the posterior dorsal and lumbar vertebra and sacrum. 
Their action is to approximate the pelvis to the thorax, and thus to oppose 
the dorsal muscles by rounding the back. 
Psoas MAGNUS is a long and strong muscle lying beneath the spine. 
Its origin is from the necks of the last two ribs, and from the bodies 
‘and transverse processes of the last dorsal and all the lumbar vertebra. 
/nsertion—into the trochanter minor internus of the femur. Action—to 
flex the haunch upon the pelvis, or, if the hind leg is fixed, to assist the 
three last muscles in rounding the back. 
Psoas parvus hes along the inner side of the P. magnus. Origin— 
from the heads of the last three ribs, and from the bodies of the three 
last dorsal and all the lumbar vertebrae. Jnsertion—into the brim of the 
pelvis. Action—to assist the last muscle in rounding the back. 
Itiacus is situated in the iliac fossa. Origin—from the crest of the 
ilium external to the sacrum, from the venter and anterior spinous pro- 
cess. Jnsertion—with the psoas magnus into the trochanter minor 
internus of the femur. Action—to flex the haunch, 
PELVIC REGION. 
SEVERAL MUSCLES are attached to the pelvis: Ist. for the control of the 
anus ; 2d. for the genital organs, and accelerating the flow of urine; 3d. 
for the movements of the tail. . 
RETRACTOR ANI is a funnel-shaped layer of thin muscular fibres arising 
within the pelvis, and inserted into the margin of the anus. Action—to 
prevent the anus from being forced outwards by the expulsive efforts of 
the abdominal muscles. 
SPHINCTER ANI is attached above to the coccyx, and encircles the anus 
with fleshy fibres, which serve to close it. 
CREMASTER is a thin layer of muscle which is attached to the yellow 
fascia covering the abdomen, and to the internal oblique, from which 
it descends upon the spermatic cord after it passes through the ex- 
ternal abdominal ring, and is inserted into the fibrous covering of the 
testicle. 
