i 7 8 HEAD AND NECK 



together so as to form a looped plexus upon the dorsum of the 

 sacrum. This communicates above with the lateral division 

 of the last lumbar nerve and below with the posterior branch 

 of the fourth sacral nerve. Branches proceed from the loops 

 to the surface of the sacro-tuberous ligament (O.T. great sacro- 

 sciatic). Finally they become superficial by piercing the 

 glutaeus maximus muscle, and they supply a limited area of 

 skin over the glutaeal region. They have already been 

 examined by the dissector of the lower limb. 



The lowest two posterior branches of the 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 artery accompany the sacral 

 nerves and anastomose with twigs from the glutaeal arteries. 



Posterior Branch 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. This plexus collects 

 blood from the integument and muscles of the back, and 

 in the thoracic and lumbar regions pours it into the posterior 

 tributaries of the intercostal and lumbar veins. In the neck 

 it is especially well marked, and its blood is emptied into the 

 vertebral veins. Not many of these venous channels will be 

 seen in an ordinary dissection. They are, however, a source 

 of serious trouble during operations upon the vertebrae. 



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, and defer the dissection of the sub- 

 occipital region until the head and neck have been removed from the body. 



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 



