THE STATUSES OF THE DURA MATER. 603 



one, generally the right, being the direct continuation of the superior longi- 

 tudinal sinus, the other of the straight sinus. They pass outward and forward, 

 describing a slight curve with its convexity upward, to the base of the petrous 

 portion of the temporal bone, then curve downward and inward on each side 

 to reach the jugular foramen, where they terminate in the internal jugular 

 vein. Each sinus rests, in its course, upon the inner surface of the occipital, 

 the posterior inferior angle of the parietal, the mastoid portion of the temporal, 

 and on the occipital again just before its termination. These sinuses are frequently 

 of unequal size, that formed by the superior longitudinal sinus being the larger, and 

 they increase in size as they proceed from behind forward. The horizontal portion 

 is of a triangular form, the curved portion semicylindrical. Their inner surface is 

 smooth, and not crossed by the fibrous bands found in the other sinuses. 

 These sinuses receive the blood from the superior petrosal sinuses at the 

 base of the petrous portion of the temporal bone, and they unite with the 

 inferior petrosal sinus, just external to the jugular foramen, to form the internal 

 jugular vein (Fig. 329). They communicate with the veins of the pericranium 

 by means of the mastoid and posterior condyloid veins, and they receive some of 

 the inferior cerebral and inferior cerebellar veins and some veins from the diploe. 

 The petro-squamous sinus, when present, runs backward along the junction of 

 the petrous and squamous-temporal, and opens into- the lateral sinus. 



The Occipital is the smallest of the cranial sinuses. It is generally single, but 

 occasionally there are two. It is situated in the attached margin of the falx cere- 

 belli. It commences by several small veins around the margin of the foramen 

 magnum, one of which joins the termination of the lateral sinus : it communicates 

 with the posterior spinal veins and terminates in the torcular Ilerophili. 



The sinuses at the base of the skull are the 



Cavernous sinuses. Superior Petrosal sinuses. 



Circular sinus. Inferior Petrosal sinuses. 



Transverse sinus. 



The Cavernous Sinuses are named from their presenting a reticulated structure, 

 due to their being traversed by numerous interlacing filaments. They are two in 

 number, of irregular form, larger behind than in front, and are placed one on each side 



y Lining membrane of sinus. 



Dura mater lininy ^ -^^^^^^^"-^ 

 pituitary fossa. ^^ _ , Fourth nerve . 



First division of fifth nerve. 

 Sixth nerve.' 



Internal carotid. 



FIG. 327. Plan showing the relative position of the structures in the right cavernous sinus, viewed from 

 behind. 



of the sella turcica, extending from the sphenoidal fissure to the apex of the petrous 

 portion of the temporal bone ; they receive anteriorly the ophthalmic vein through 

 the sphenoidal fissure, and open behind into the petrosal sinuses. On the inner 

 wall of each sinus is found the internal carotid artery, accompanied by filaments of 

 the carotid plexus and by the sixth nerve ; and on its outer wall, the third, fourth, 

 and ophthalmic division of the fifth nerve. These parts are separated from the 

 blood flowing along the sinus by the lining membrane, which is continuous with the 

 inner coat of the veins. The cavernous sinuses receive some of the cerebral veins, 

 and also a small sinus, the spheno-parietal, which extends inward on the under 

 aspect of the lesser wing of the sphenoid ; they communicate with the lateral 

 sinuses by means of the superior and inferior petrosal sinuses, and with the facial 

 vein through the ophthalmic vein. They also communicate with each other by 

 means of the circular sinus. 



