SINUSES OF THE DURA MATER. 543 



terminates in the straight sinus. It receives several veins from the falx cerebri, 

 and occasionally a few from the flat surface of the hemispheres. 



The Straight Sinus is situated at the line of junction of the falx cerebri with 

 the tentorium. It is triangular in form, increases in size as it proceeds back- 

 wards, and runs obliquely downwards and backwards from the termination of 

 the inferior longitudinal sinus to the torcular Herophili. Besides the inferior 

 longitudinal sinus, it receives the venae Galeni, the inferior median cerebral 

 veins, and the superior cerebellar. A few transverse bands cross its interior. 



The Lateral Sinuses are of large size, and are situated in the attached margin 

 of the tentorium cerebelli. They commence at the torcular Herophili, and 

 passing horizontally outwards to the base of the petrous portion of the temporal 

 bone, curve downwards and inwards 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, and 

 they increase in size as they proceed from behind forwards. The horizontal 

 portion is of a triangular form, the curved portion semi-cylindrical ; their inner 

 surface is smooth, and not crossed by the fibrous bands found in the inner 

 sinuses. These sinuses receive blood from the superior longitudinal, the 

 straight, and the occipital sinuses; and in front they communicate with the 

 superior and inferior petrosal. They communicate with the veins of the peri- 

 cranium by means of the mastoid and posterior condyloid veins, and they re- 

 ceive the inferior cerebral and inferior cerebellar veins, and some veins from 

 the diploe. 



The Occipital are the smallest of the cranial sinuses. They are usually two 

 in number, and situated in the attached margin of the falx cerebelli. They 

 commence by several small veins around the posterior margin of the foramen 

 magnum, which communicate with the posterior spinal veins, and terminate by 

 separate openings (sometimes by a single aperture) in the torcular Herophili. 



The sinuses at the base of the skull are : 



Cavernous. Inferior petrosal. 



Circular. Superior petrosal. 



Transverse. 



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

 They are two in number, of large size, and placed one on each side of the sella 

 Turcica, extending from the sphenoidal fissure to the apex of the petrous por- 

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

 the sphenoidal fissure, and communicate behind with the petrosal sinuses, and 

 with each other by the circular and transverse 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 nerves. 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 cavity of the sinus, which is larger behind than in front, is 

 intersected by filaments of fibrous tissue and small vessels. The cavernous 

 sinuses receive the inferior anterior cerebral veins; they communicate with the 

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

 facial vein through the ophthalmic. 



The ophthalmic is a large vein, which connects the frontal vein at the inner 

 angle of the orbit with the cavernous sinus ; it pursues the same course as the 

 ophthalmic artery, and receives branches corresponding to those derived from 

 that vessel. Forming a short single trunk, it passes through the inner ex- 

 tremity of the sphenoidal fissure, and terminates in the cavernous sinus. 



The Circular Sinus completely surrounds the pituitary body, and commu- 

 nicates on each side with the cavernous sinuses. Its posterior half is larger 



