The posterior primary division of the first cervical or suboccipital nerve springs from the trunk, between the vertebral artery and the posterior arch of the atlas, passes dorsalward into the sub-occipital triangle, and breaks up into branches which supply the superior oblique, the inferior oblique, and the major rectus capitis posterior muscles, which form the lateral boundaries of the triangle.
It also gives a branch across the posterior surface of the major rectus capitis posterior to the minor rectus capitis posterior, and a branch to the semispinalis capitis (complexus) in the roof of the triangle.
It communicates with the medial branch of the posterior primary division of the second cervical nerve, either through or over the inferior obUque muscle, and it occasionally gives a cutaneous branch to the skin of the upper part of the back of the neck and the lower part of the scalp.
The posterior primary division of the second cervical nerve is the largest posterior division of all the cervical nerves. It divides into a small lateral branch and a very large medial branch. The lateral branch gives a twig to the inferior oblique and terminates in branches which supply the splenius and longissimus capitis (trachelo-mastoid) muscles. The medial branch is the greater occipital nerve. It turns around the lower border of the inferior oblique, crosses the sub-occipital triangle obliquely, pierces the semispinalis capitis (complexus), the tendon of the trapezius, and the deep cervical fascia, passing through the latter immediately below the superior nuchal line of the occipital bone, and it divides into several terminal sensory branches which ramify in the superficial fascia of the scalp.
It gives one or two motor twigs to the semispinalis capitis (complexus), and its terminal branches which are accompanied by branches of the occipital artery supply the skin of the scalp, above the superior nuchal Une, as far forward as the vertex. Occasionally one branch reaches the pinna and supplies the skin on the upper part of its medial aspect. As it turns around the inferior oblique it gives branches which join with the medial branches of the posterior primary divisions of the first and third cervical nerves, and in this manner a small looped plexus is formed beneath the semispinalis capitis (complexus) muscle, the posterior cervical plexus of Cruveilhier.
The posterior primary branches of the third, fourth, and fifth cervical nerves divide at the lateral border of the semispinalis colli into medial and lateral branches. The medial branches of the third, fourth, and fifth nerves run backward between the semispinalis colli and capitis (complexus), supplying both muscles. Then, after passing backward between the semispinalis capitis and the ligamentum nuchse, they pierce the origin of the trapezius and supply the skin of the back of the neck. The greater part of the medial branch of the third nerve, which runs upward in the superficial fascia to the scalp, is called the third or smallest occipital nerve ; it interlaces with the greater occipital nerve, and it supplies the skin of the upper part of the back of the neck, near the middle line, and the skin of the scalp in the region of the external occipital protuberance.
The medial branches of the posterior primary divisions of the sixth, seventh, and eighth cervical nerves pass to the median side of the semispinalis colli, between it and the subjacent multifidus spinse, and they end in the neighbouring muscles. The lateral branches of the posterior primary divisions of the last five cervical nerves are small and they are distributed to the longissimus capitis (trachelo-mastoid), the ilio-costalis cervicis (cervicalis ascendens), the longissimus cervicis (transversalis cervicis), the semispinalis capitis (complexus), and the splenius muscles.
