908 THE NERVOUS SYSTEM 



the vertebral artery and the posterior arch of the atlas, passes backwards into the 

 sub-occipital triangle, and breaks up into branches which supply the superior 

 oblique, the inferior oblique, and the rectus capitis posterior major muscles, 

 which form the lateral boundaries of the triangle. It also gives a branch across 

 the posterior surface of the rectus capitis posterior major to the rectus capitis 

 posterior minor, 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 oblique 

 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. 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 branches which ramify 

 in the superficial fascia of the scalp. It gives one or two motor twigs to the semi- 

 spinalis 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 

 line, as far forwards 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 in- 

 ferior oblique it gives communicating branches to the medial branches of the pos- 

 terior 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 outer border of the semispinalis colli into medial and lateral 

 branches. The medial branches of the third, fourth, and fifth nerves run back- 

 wards between the semispinalis colli and capitis (complexus), supplying both muscles. 

 Then, after passing backwards 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 upwards 

 in the superficial fascia to the scalp, is called the third or smallest occipital nerve; 

 it communicates 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 spina?, 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-mas- 

 toid), the ilio-costalis cervicis (cervicalis ascendens), the longissimus cervicis (trans- 

 versalis cervicis), the semispinalis capitis (complexus), and the splenius muscles. 



THORACIC NERVES 



The posterior primary divisions of all the thoracic nerves divide into medial 

 and lateral branches as soon as they enter the vertebral groove. The medial 

 branches of the upper six thoracic nerves pass backwards between the semi- 

 spinalis dorsi and the multifidus spinse; they supply the spinalis dorsi, the semi- 

 spinalis dorsi, the multifidus spinge, the rotatores spinse, the intertransversales, and 

 the interspinales muscles, and they end in cutaneous branches which, after piercing 

 the trapezius, turn lateralwards in the superficial fascia of the back, and supply the 

 skin as far as the middle of the scapula. The cutaneous branch of the second 

 nerve is the largest ; it can be traced lateralwards as far as the acromion process. 

 The medial branches of the lower six thoracic nerves run dorsalwards, between 

 the longissimus dorsi and the multifidus spinse; they chiefly end in twigs to the adja- 

 cent muscles, but not uncommonly they give small cutaneous twigs which pierce the 

 latissimus dorsi and the trapezius and end in the skin near the mid-line of the back. 



