74 HEAD AND NECK 



region. They have already been examined by the dissector 

 of the lower limb. 



The posterior rami of the loivest two sacral nerves do 

 not separate into medial and lateral divisions. They are 

 very small, and, after communicating with each other, and 

 also with the coccygeal nerve^ they distribute filaments to the 

 parts on the posterior aspect of the lower portion of the 

 sacrum and on the dorsal aspect of the coccyx. 



Twigs from the lateral sacral arteries accompany the sacral 

 nerves and anastomose with twigs from the gluteal arteries. 



Posterior Ramus of the Coccygeal Nerve. This is a slender 

 twig which emerges from the inferior opening of the sacral 

 canal, and, after being joined by a filament from the last 

 sacral nerve, is distributed on the dorsum of the coccyx. 



Posterior Vertebral Venous Plexus. A plexus of veins is 

 situated upon the superficial aspect of the vertebral arches 

 subjacent to the multifidus muscle. Blood passes to it from 

 the integument and muscles of the back, and is conveyed by 

 it, in the thoracic and lumbar regions, into the posterior 

 tributaries of the intercostal and lumbar veins. In the neck 

 it is especially well marked, and there blood is emptied from 

 it into the vertebral veins. In an ordinary dissection, the 

 plexus is not very noticeable, but it is a source of serious 

 trouble during operations upon the vertebrae (comp. p. 79). 



Dissection. The fourth day after the body is placed upon 

 its face must be devoted to the dissection of the sub-occipital 

 triangle, and the fifth day to the display of the medulla spinalis 

 (O.T. spinal cord), its membranes, nerve-roots, and blood vessels. 



If the dissector is pushed for time, it is better that he should 

 proceed at once to expose the spinal medulla (p. 78), and defer 

 the dissection of the sub-occipital region until the head and neck 

 have been removed from the trunk. 



Sub-Occipital Space. The sub-occipital space is a small 

 triangular area, exposed by the reflection of the semi- 

 spinalis capitis (O.T. complexus) and the splenius muscle. 

 It is bounded by three muscles (i) the rectus capitis 

 posterior major forms its upper and medial boundary ; 

 (2) the obliquus inferior limits it below ; and (3) the obliquus 

 superior bounds it above and to the lateral side. Its 

 floor consists of two structures viz., the posterior arch of 

 the atlas and the thin posterior atlanto-occipital membrane. 

 It contains a portion of the vertebral artery and the posterior 

 ramus of the sub-occipital or first cervical nerve (Fig. 20). 



