974 THE VASCULAE SYSTEM. 



lateral sinus of the opposite side. Opening into the superior sagittal sinus are the 

 superior cerebral veins, and it communicates on each side by small openings with 

 a series of spaces in the dura mater, the lacunae laterales, into which the arach- 

 noideal granulations project. It also communicates, by emissary veins, which pass 

 through the foramen caecum and through each parietal foramen, with the veins on 

 the exterior of the cranium. Its cavity, which is triangular in transverse section, 

 is crossed by several fibrous strands called the chordae Willisii. 



Sinus Sagittalis Inferior. The inferior sagittal sinus lies, usually, in the posterior 

 two-thirds of the lower free margin of the falx cerebri. It terminates posteriorly 

 by joining with the great cerebral vein (Galen) to form the straight sinus. It 

 receives tributaries from the falx cerebri and from the medial surface of the 

 middle third of each cerebral hemisphere. 



Sinus Intercavernosi. The anterior intercavernous sinus is a small transverse 

 channel which crosses from one cavernous sinus to the other in the anterior border 

 of the diaphragma sellse. 



The posterior intercavernous sinus also connects the two cavernous sinuses 

 together. It lies in the posterior border of the diaphragma sellse. 



The anterior and posterior intercavernous sinuses and the intervening parts of 

 the cavernous sinuses form collectively the circular sinus. 



Plexus Basilaris. The basilar plexus (O.T. basilar sinus) is situated in the dura 

 mater on the basilar part of the occipital bones. It connects the posterior ends 

 of the cavernous or the anterior ends of the inferior petrosal sinuses together, and 

 communicates below with the anterior spinal veins. 



Sinus Rectus. The straight sinus is formed by the union of the inferior 

 sagittal sinus with the great cerebral vein (Galen). It runs downwards and 

 backwards, along the line of union between the falx cerebri and the tentorium 

 cerebelli. As a general rule it turns to the left at the internal occipital protuber- 

 ance, dilates somewhat, and becomes continuous with the left transverse sinus, 

 its dilatation being united with the corresponding dilatation on the lower end of 

 the superior sagittal sinus, the " confluens sinuum," by a transverse anastomosing 

 channel. Occasionally the straight sinus terminates in the right lateral sinus; 

 in that case the superior sagittal sinus ends in the left transverse sinus ; and 

 sometimes it bifurcates to join both transverse sinuses. It receives some of the 

 superior cerebellar veins and a few tributaries from the falx cerebri. 



Paired Sinuses. There are six pairs of sinuses, viz., the transverse, the occipital, 

 the cavernous, the superior petrosal, the inferior petrosal, and the spheno-parietal. 



Sinus Transvessi (O.T. Lateral Sinuses). Each transverse sinus commences 

 at the internal occipital protuberance, the right usually as the continuation of the 

 superior sagittal, and the left as the continuation of the straight sinus. Each 

 passes laterally in the postero-lateral part of the attached border of the tentorium 

 cerebelli and in a groove in the occipital bone. From the lateral angle of the 

 occipital bone it passes to the posterior inferior angle of the parietal bone, which 

 it grooves ; then it leaves the tentorium and turns downwards on the inner surface 

 of the mastoid portion of the temporal bone ; from the latter it passes to the upper 

 surface of the jugular process of the occipital bone, and turns forwards and then 

 downwards into the jugular foramen, where it becomes continuous with the internal 

 jugular vein. The part which descends on the temporal bone and turns forwards 

 on the jugular process of the occipital is called the sigmoid sinus. 



Its tributaries are some of the superior and inferior cerebellar veins, a posterior 

 diploic vein, and the superior petrosal sinus. It is connected with the veins out- 

 side the cranium by emissary veins wjiich pass through the mastoid foramen and 

 the condyloid canal. 



Sinus Occipitales. The occipital sinuses lie in the attached border of the 

 falx cerebelli and in the dura mater along the postero-lateral boundaries of the 

 foramen magnum ; frequently they unite above and open by a single channel into 

 the commencement of either the right or the left transverse sinus, but their upper 

 extremities may remain separate, and then each communicates with the commence- 

 ment of the transverse sinus of its own side. On the other hand either the right or 

 the left sinus may be absent. Each opens below into the terminal part of the corre- 



