The intrinsic back muscles are subdivided into 3 layers: the superficial layer, the intermediate layer, and the deep layer. The superficial layer of the intrinsic back muscles contains the uppermost deep back muscles, which are the splenious muscles—the splenius cervicis and splenius capitis.
In contrast to extrinsic muscles, the intrinsic back muscles, which are located more deeply, are the muscles that act on the spine and head, as opposed to the ribs and upper limb.
As the name suggests, the superficial layer of the intrinsic back muscles sits on top of the other 2 layers in this group. The superficial layer is comprised entirely of the splenius muscles — splenius cervicis and splenius capitis. Together the splenius muscles cover the vertically oriented paraspinals (which are deeper and comprise the intermediate layer of the intrinsic back muscles.)
The term splenius is taken from the Latin word for bandage, which is splenion. And in a sense, the splenius muscles appear as if they bandage the paraspinals and the vertically oriented muscles that make up the deepest intrinsic layer. The splenius muscles serve to hold these deeper layers in position.
The splenius capitis is positioned above the splenius cervicis and these muscles act together. The splenius cervicis originates at the spinous processes of T3-T6, and the splenius capitis originates at the spinous processes of C3-T3.
These muscles cover the area from the bottom of the neck (C-7) down to the upper part of the thoracic spine (T-3 or T-4.) They start at the center of the spine and together, they form a “V” shape. The sides of the “V” are very thick and the central indentation is quite shallow.
The individual muscle fibers of the splenius capitis and cervicis orient on a diagonal. Beginning at the midline of the spine, the fibers angle up to the skull and cervical vertebrae. The insertion of splenius cervicis is the transverse process of C1 and C2, and the insertion of the splenius capitis is the lateral superior nuchal line and the mastoid process.
The splenius capitus flexes and rotates your neck.
The splenius capitis muscle starts at the midline of the spine at C3 to T3, spanning the levels between your 7th cervical vertebra to your 3rd or 4th (it varies) thoracic vertebrae. This muscle inserts at the nuchal ligament, a very strong ligament of the neck.
The splenius capitis muscle then angles up and out to attach to your skull at 2 places: the mastoid process, which is located behind and towards the very bottom of the ear, and at the nuchal line of the skull, which is at the back of your skull but at a level higher than the mastoid process.
The splenious cervicis supports your neck, flexes it and moves it laterally to the side.
Like the splenius capitis, the splenius cervicis starts at the midline of the spine, originating in spinous processes of T3-T6, and spanning the levels between your 7th cervical vertebra to your 3rd or 4th (it varies) thoracic vertebrae.
The splenius cervicis inserts onto the transverse processes of C1 and C2. When both sides of the splenius machine act together, the result is neck extension, which equates to bringing the head back toward the back of the neck. When only one side is contracting, the splenius muscles help tilt and/or rotate the neck to the side of the contraction.