This is the longest, the largest, and most powerful muscle in the body. It is situated on the arches of the ribs, in close connection with the spines of the dorsal and lumbar vertebrae, and extends from the ilium behind, over the loins and the back to the fourth cervical vertebra in front.

Origin. - From the internal surface and anterior border of the ilium or haunch bone, and the ligament (sacro-iliac) connecting it with the sacrum, and from the sacral bone.

Insertion. - It has numerous connections in its course towards the neck..

1. To the spinous, transverse, and articular processes of the lumbar vertebrae.

2. To the spinous and transverse processes of the dorsal vertebrae. 3. To the external surfaces of the 14th and 15th posterior ribs. 4. To the spinous processes of the last four cervical vertebrae.

Action. - Acting with its fellow on the opposite side, it extends the spine. By its attachments to the cervical vertebrae it raises and supports the neck, and by its connection with the ribs it assists in expiration. It is the principal muscle concerned in rearing, kicking, and jumping.