Loin

The lumbar region, or loins, is the part of the trunk located between the rib cage and the pelvis. It is bounded dorsally by the lumbar vertebrae and ventrolaterally by the abdominal wall, which is composed of four main muscles (external oblique, internal oblique, transversus abdominis, and rectus abdominis).

This region supports the abdominal cavity, protects vital organs such as the kidneys, and plays a fundamental role in locomotion by transmitting forces from the hind limbs to the rest of the body. Its skeletal framework consists of the lumbar vertebrae, which are characterized by large vertebral bodies and long transverse processes that serve as attachment sites for the lumbar muscles. The lumbar spinal nerves emerge from this region to form part of the lumbosacral plexus, which innervates the hind limbs.

The number of lumbar vertebrae varies among species: dogs and cats have seven (L7), while horses and cattle have six (L6), and pigs have six or seven (L6–7). In horses, the transverse processes are particularly wide, which limits lateral flexion. Clinically, the thoracolumbar junction (T3–L3) is a frequent site of spinal cord injury that can cause paresis or paralysis of the hind limbs. In humans, the lumbar region typically consists of five lumbar vertebrae (L5), which are the largest and strongest in the vertebral column, supporting a significant portion of the body’s weight.