Original Editor - Venus Pagare
Musculus splenius capitis is one of the deep (or intrinsic) muscles of the back. It is a broad strap-like muscle in the back of the neck. Deep to Sternocleidomastoid at the mastoid process.
Dorsal ramus of spinal nerves C3-C6
Muscular branches of the occipital artery from the external carotid artery. 
Acting bilaterally: extension of the head and neck;
Acting unilaterally: lateral flexion of the head and neck and rotation the head to the same side. 
It also assists in supporting the head in the erect position
- Gray's Anatomy of Human Body, Bartleby.com edition. New York: 2000.fckLRhttp://www.bartleby.com/107/115.html