Obliquus capitis inferior muscle

The obliquus capitis inferior muscle is a muscle in the upper back of the neck. It is one of the suboccipital muscles. Its inferior attachment is at the spinous process of the axis; its superior attachment is at the transverse process of the atlas. It is innervated by the suboccipital nerve (the posterior ramus of first cervical spinal nerve). The muscle rotates the head to its side.

Despite what its name suggest, it is the only capitis (Latin: "head") muscle that does not actually attach to the skull.

Anatomy
The obliquus capitis inferior is one of the suboccipital muscles (and the only one of these to have no attachment to the skull). It is larger than the obliquus capitis superior muscle. It forms the inferolateral boundary of the suboccipital triangle.

The muscle extends laterally and somewhat superiorly from its inferior attachment to its superior attachment.

Attachments
its inferior attachment is at the lateral external aspect of the bifid spinous process  of the axis (cervical vertebra C2)   (inferior to the attachment of the rectus capitis posterior major muscle ) and the lamina of the axis.

Its superior attachment is at (the inferoposterior aspect of ) the transverse process of the atlas (cervical vertebra C1).  

Innervation
The muscle receives motor innervation from the suboccipital nerve (the posterior ramus of cervical spinal nerve C1). 

Relations
It lies deep to the semispinalis capitis and trapezius muscles.

Actions/movements
The muscle acts to rotate the atlas  (and thus  the head) ipsilaterally.  It acts together with the rectus capitis posterior major muscle. 

Function
The muscle is responsible for rotation of the head and first cervical vertebra (atlanto-axial joint). 

The obliquus capitis inferior muscle, like the other suboccipital muscles, has an important role in proprioception. This muscle has a very high density of Golgi organs and muscle spindles which accounts for this. It is believed that proprioception may be the primary role of the inferior oblique (and indeed the other suboccipital muscles), allowing accurate positioning of the head on the neck.