LOWER SINUSES OF TPIE DUE A MATER. 



483 



from the straight and occipital sinuses, from the veins upon the sides 

 and base of the brain, from those on the under surface of the cere- 

 bellum, and from some of the veins of the diploe. The petrosal sinuses 

 also join the lateral sinus on each side : and two emissar/j xein?, connect 

 these with the veins at the back of the head and neck. 



Fig. 307. — Sketch of the Venous Sinuses Fig- 807. 



IN THE Base of the Cranium, with the 



OriiTHALMic Vein. 



a, sella turcica and circular sinns ; b, 

 cavernous sinus receiving c, the ophthalmic 

 vein ; d, superior petrosal ; c, inferior petrosal 

 sinus ; /, transverse siniis ; ff, occipital ; 7i, 

 lateral ; i, termination of the superior longi- 

 tudinal in the torcular Herophili. 



The posterior occipital sirnis is 



vsoraetimes a single canal, at other 

 times double, as if composed of 

 two compartments. It lies along 

 the attached border of the falx 

 cerebelli, extending from the pos- 

 terior margin of the foramen magnum 

 to the confluence of the sinuses. 

 It communicates in front with the 

 posterior spinal plexuses of veins. 



The sinuses placed at the base of 

 the skull are as follows, taking them in their order from before backwards. 



The circular sinus has the form of a ring, and is placed super- 

 ficially in the substance of the dura mater round the pituitary body ; it 

 receives the blood from the minute veins of the pituitary body and 

 communicates at each side with the cavernous sinus. Sometimes it is 

 only partially developed, the part in front of the gland being that 

 usually present ; sometimes, however, it is behind the gland. 



The cavernous sinuses, placed one on each side of the body of the 

 sphenoid bone, over the roots of the great wings, and stretching from 

 the sphenoidal fissure to the apex of the petrous portion of the tem- 

 poral bones, are of considerable size, and of very irregular form. Each 

 receives the ophthalmic vein at its fore part, and communicates in- 

 ternally with the circular sinus, and posteriorly with the petrosal 

 sinuses. In the wall of each, separated by the lining membrane from 

 the cavity of the sinus, pass forward the third, fourth, and sixth 

 cranial nerves, the ophthalmic division of the fifth nerve, and the 

 internal carotid artery. 



The superior petrosal sinus is a narrow canal running along the 

 upper margin of the petrous part of the temporal bone. Commencing 

 at the back part of the cavernous sinus, it is directed outwards and 

 backwards in the attached margin of the tentorium cerebelli ; and, 

 descending a little, ends in the lateral sinus where this lies upon the 

 temporal bone. 



The inferior petrosal sinus, wider than the upper, passes downwards 

 and backwards along the lower margin of the petrous bone, between 

 this and the basilar process of the occipital bone. It opens into the 

 lateral sinus near its termination, or into the internal jugular vein. 



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