The spinous process of a vertebra is directed backward and downward from the junction of the laminae

Lamina of the vertebral arch

The laminæ are two broad plates, extending dorsally and medially from the pedicles, fusing to complete the roof of the vertebral arch.Their upper borders and the lower parts of their anterior surfaces are rough for the attachment of the ligamenta flava....

 (in humans), and serves for the attachment of muscle

Muscle

Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

s and ligaments. In animals without an erect stance, the process points upward and may slant forward or backward. Spinous processes are exaggerated in some animals, such as the extinct Dimetrodon

Dimetrodon

Dimetrodon was a predatory synapsid genus that flourished during the Permian period, living between 280–265 million years ago ....

and Spinosaurus

Spinosaurus

Spinosaurus is a genus of theropod dinosaur which lived in what is now North Africa, from the lower Albian to lower Cenomanian stages of the Cretaceous period, about 112 to 97 million years ago. This genus was first known from Egyptian remains discovered in 1912 and described by German...

, where they form a sail- or finback.