MENINGEAL ARTERIES. 29 



trunk of the internal carotid artery surrounded by the sympathetic, with 

 the sixth nerve on the outer side of the vessel ; but all these are shut out 

 from the blood in the space by a thin lining membrane. 



The cavernous sinus receives the ophthalmic vein of the orbit, some 

 inferior cerebral veins, and twigs from the pterygoid veins outside the 

 skull. It communicates with its fellow on the opposite side by the circular 

 and transverse sinuses ; and its blood is transmitted to the lateral sinus by 

 the superior and inferior petrosal channels. 



The circular sinus lies around the pituitary body, and reaches from the 

 one cavernous sinus to the other across the middle line. Besides serving 

 as the means of communication between those sinuses, it receives small 

 veins from the pituitary body. This sinus is usually destroyed by the 

 removal of the pituitary body. 



The transverse or basilar sinus crosses the basilar process of the occipi- 

 tal bone, on a level with the petrous part of the temporal bone, and unites 

 the opposite cavernous sinuses. A second transverse sinus is sometimes 

 found nearer the foramen magnum. 



The superior petrosal sinus (fig. 3,/) lies in a groove in the upper 

 edge of the petrous part of the temporal bone, and extends between the 

 cavernous and lateral sinuses. A small vein from the cerebellum, and an- 

 other from the internal ear, are received into it. 



The inferior petrosal sinus (fig. 3, /?) extends between the same sinuses 

 as the preceding, and lies in a groove along the line of junction of the 

 petrous part of the temporal with the basilar process of the occipital bone; 

 it is joined by a small vein from the outside of the skull, through the fora- 

 men lacerum in the base of the cranium. This sinus passes through the 

 anterior compartment of the jugular foramen, and ends in the internal 

 jugular vein. 



MENINGEAL ARTERIES. These arteries supplying the cranium and the 

 dura mater come through the base of the skull ; they have been named 

 from their situation in the three fossae, anterior, middle, and posterior 

 meningeal. 



The anterior meningeal are very small branches of the ethmoidal arteries 

 (p. 56), which enter the skull by apertures between the frontal and eth- 

 moid bones ; they are distributed to the dura mater over and near the 

 ethmoid bone. 



The middle meningeal arteries are three in number : two, named large 

 and small, are derived from the internal maxillary trunk ; and the third is 

 an offset of the ascending pharyngeal artery. 



a. The large meningeal branch of the internal maxillary artery appears 

 through the foramen spinosum of the sphenoid bone, and ascends towards 

 the anterior inferior angle of the parietal bone. At this spot the vessel 

 enters a deep groove in the cranium, and ends in ramifications which 

 spread over the side of the head, some of them reaching to the top and the 

 occiput, whilst others perforate the bone, and end on the exterior of the 

 head. Two veins accompany the artery. 



Branches. As soon as the artery comes into the cranial cavity, it 

 furnishes branches to the dura mater and osseous structure, and to the 

 ganglion of the fifth nerve. One small offset, petrosal, enters the hiatus 

 Fallopii, and supplies the surrounding bone (Hyrtlj. One or two branches 

 pass into the orbit, and anastomose with the ophthalmic artery. 



b. The small meningeal branch is an offset of the large one outside the 



