io66 A MANUAL OF ANATOMY 



posterior occipito-atlantal ligament. It leaves the suboccipital 

 triangle by piercing the duca mater, and enteis the cranial cavity 

 through the foramen magnum. The third part of the vessel fur- 

 nishes muscular branches to the adjacent muscles which anastomose 

 with the arteria princeps cervicis of the occipital, and the deep 

 cervical of the superior intercostal. 



Suboccipital Plexus of Veins. — ^This plexus is formed by branches 

 which proceed from (i) the intraspinal venous plexus, (2) the ad- 

 jacent muscles, and (3) the occipital periosteum. It receives the 

 internal occipital vein, and the blood is conveyed away from it by 

 the deep cervical vein, already described, and the vertebral vein, 

 which will be afterwards considered. 



Posterior Primary Division of the Suboccipital Nerve. — ^This divi- 

 sion passes backwards, as a single nerve, in the vertebrarterial 

 groove of the atlas, lying between the bone and the third part of 

 the vertebral artery. On entering the suboccipital triangle it 

 divides into branches which supply the obliqui, recti postici, and 

 complexus muscles. 



Posterior Primary Divisions of the Cervical Spinal Nerves. — The 

 cervical nerves are eight in number, the first being known as the 

 suboccipital nerve. Each breaks up into an anterior and a pos- 

 terior primary division. The posterior primary division of the 

 first or suboccipital is single, and has just been described. The 

 posterior primary divisions of the lower seven arise in the inter- 

 vertebral foramina, and each soon divides into an external and 

 internal branch. The external branches of all are distributed to 

 the deep muscles, and do not become cutaneous. The internal 

 branch of the posterior primary division of the second, which is of 

 large size, constitutes the great occipital nerve. It winds round 

 the lower border of the obliquus capitis inferior, and pierces the 

 complexus, to which it gives branches. Thereafter it may pierce 

 the trapezius to reach its cutaneous distribution, which has been 

 already described. The internal branches of the third, fourth, and 

 fifth are directed inwards upon the semispinalis colli, to which, as 

 well as to the complexus, they give branches. Close to the cervical 

 spinous processes they pass backwards, internal to the biventer 

 cervicis (part of the complexus), and, having pierced the trapezius, 

 they reach the integument, which they supply. The internal 

 branch of the posterior primary division of the third, before piercing 

 the trapezius, gives off a small ascending branch, known as the 

 least or third occipital. It passes upwards, lying at first under 

 cover of the trapezius, and then passing through it, to reach the 

 occipital integument. The internal branches of the posterior 

 primary divisions of the sixth, seventh, and eighth are directed in- 

 wards towards the cervical spinous processes beneath the semi- 

 spinalis colli, and are distributed to the adjacent muscles. They 

 do not furnish any branches to the integument. 



