VENOUS SINUSES OF THE DURA MATER 



651 



transverse sinus, run around the foramen magnum, where they communicate with 

 the venous vertebral retia, and unite at a variable distance from the internal oc- 

 cipital protuberance to form the single occipital sinus. Sometimes they re- 

 main separate as far as the occipital protuberance, then forming two occipital 

 sinuses. One of the two marginal sinuses may be much smaller than the other, 

 or be entirely absent. At the point where the marginal sinuses unite to form the 

 single occipital sinus, there is a communication with the venous vertebral retia. 

 The occipital sinus ends in the confluens sinuum. It receives in its course veins 

 from the tentorium cerebelli, and from the inferior surface of the cerebellum. 

 It communicates- through the plexus of veins which surrounds the hypoglossal 

 nerve [rete canalis hypoglossi] in the hypoglossal (anterior condyloid) canal 

 with the vertebral vein and the longitudinal vertebral venous sinuses. 



(5) The circular sinus [sinus circularis] (fig. 516) encircles the hypophysis 

 cerebri. It consists of the two cavernous sinuses and their communications across 



Fig. 515. — The Venous Sinuses. (Longitudinal section.) 

 Abducens nerve Oculomotor nerve 



Falx cerebri 



Optic nerve 



Superior sagit- 

 tal sinus 



Inferior sagital 

 sinus 



Vein of Galen 



Straight sinus 



Tentorium 



cerebelli 



Transverse 



sinus 



Superior 

 petrosal sinus 



Falx cerebelli 



Facial and auditory ,,^''^^ 

 nerves 

 Glossopharyngeal, vagus and 

 accessory nerves 



Hypoglosaal nerve 



Vertebral 

 artery 



Second cervical nerve 



Ligamentum denticulatum 



Inferior petrosal sinus 



the median line by means of the anterior and posterior intercavernous sinuses. 

 The intercavernous sinuses are small and cross the median line in front of and 

 behind the hypophysis, respectively. 



(6) The basilar plexus [plexus basilaris] is a venous plexus in the substance of 

 the dura mater over the basilar part of the occipital bone. It extends from 

 the cavernous sinus to the margin of the foramen magnum below. It communi- 

 cates laterally with the inferior petrosal sinus, and inferiorly with the internal 

 vertebral venous plexuses. One of the larger of the irregular venous channels 

 forming the plexus passes transversely from one inferior petrosal sinus to the 

 other. This venous plexus is serially homologous with the longitudinal vertebral 

 venous sinuses on the posterior surfaces of the bodies of the vertebrae. 



(7) The transverse (or lateral) sinus [sinus transversus] (figs. 514, 516) 

 extends from the internal occipital protuberance to the jugular foramen. In 

 this course it lies in the groove (which has been named after it) along the squamous 

 portion of the occipital bone, the posterior inferior angle of the parietal bone, the 

 mastoid portion of the temporal bone, and the jugular process of the occipital 

 bone. It at first runs laterally and forward horizontally between the two layers 

 of the tentorium cerebelli, following the curve of the groove on the occipital and 

 on the mastoid angle of the parietal bone. On reaching the groove in the mas- 



