i8 2 HEAD AND NECK 



meninges, the bones, the periosteum, and the ligaments ; and 

 their arrangement is very much the same in each of the 

 three regions. 



Each spinal artery may be looked upon as giving off three 

 main twigs: of these, one, termed the pre-laminar branch, 

 a very small twig, ramifies upon the deep surface of the 

 vertebral arches and ligamenta flava ; another, the neural 

 branch, can be followed to the dura mater, which it pierces 

 immediately above the point of exit of the corresponding 

 spinal nerve; whilst the third, fat post-central branch, is carried 

 medially, anterior to the dura mater, towards the posterior 

 surface of the vertebral bodies, and divides into an ascending 

 and a descending twig. These anastomose with the correspond- 

 ing twigs above and below, and in this manner a continuous 

 series of minute arterial arcades is formed, from which 

 branches pass medially to form a series of cross anastomoses 

 with the corresponding vessels of the opposite side. 



In the cervical region small branches from the ascending 

 cervical artery also find their way into the vertebral canal ; 

 whilst in the sacral portion of the canal the dissector will find 

 branches from the lateral sacral arteries. 



Internal Vertebral Venous Plexus. This plexus extends 

 along the whole length of the vertebral canal, and consists 

 essentially of four subsidiary longitudinal plexuses, two 

 anterior and two posterior, which anastomose freely with 

 each other. 



The posterior plexuses are united by many cross branches, 

 which run along the deep aspect of the vertebral arches and 

 ligamenta flava. Above, they communicate with the occipital 

 sinus, whilst all the way down they are connected with the 

 posterior vertebral venous plexus by wide channels which pierce 

 the ligamenta flava. Laterally they send branches through 

 the intervertebral foramina to join the posterior branches of 

 the intercostal and lumbar veins. 



The anterior plexuses cannot be dissected whilst the medulla 

 spinalis (O.T. spinal cord) and its membranes are in situ, but 

 it is convenient to describe them at this stage. Indeed, the 

 dissection is one of considerable difficulty, even under the 

 most advantageous circumstances. They form two main 

 longitudinal venous channels placed one upon either side of 

 the posterior longitudinal ligament of the vertebral bodies, and 

 they are joined by transverse branches which cross the median 



