REMOVAL OF THE BRAIN 219 



interior of the brain to enter the straight sinus, is ruptured by this 

 proceeding. 



Meningeal Veins. In addition to the named blood sinuses, venous 

 channels accompany the meningeal arteries and more particularly the 

 trunks and branches of the middle meningeal artery. These vessels are 

 of wider calibre than the corresponding arteries, and lie external to them 

 in the grooves on the inner surfaces of the cranial bones. When the 

 arteries are distended they compress the middle parts of the veins and 

 drive the blood into their anterior and posterior margins. When this 

 occurs each artery appears to be accompanied by two veins, a circumstance 

 which is probably responsible for the statement that some of the meningeal 

 arteries have venae comites. 



Emissaria. Emissary veins are blood channels which 

 connect the sinuses of the dura mater with the veins which 

 lie outside the cranium. They are: (i) Emissary veins con- 

 nected with the superior sagittal sinus (a) from the anterior 

 extremity of the sinuses an emissary vein passes through 

 the foramen caecum. This vein divides below and either 

 becomes continuous with the veins of the nasal fossae, or its 

 branches pass through foramina in the nasal bones and join 

 the angular veins ; (b) two parietal emissary veins, which 

 pass through the parietal foramina and connect the superior 

 sagittal sinus with the occipital veins. (2) Emissary veins 

 connected with the transverse sinuses (a) two mastoid 

 emissary veins, one on each side, pass through the mastoid 

 foramina and connect the sigmoid parts of the transverse 

 sinuses with the posterior auricular veins ; (b) two post- 

 condyloid veins, one on each side, pass through the condyloid 

 canals and connect the lower ends of the transverse sinuses 

 with the plexuses of veins in the suboccipital triangles. (3) 

 Emissary veins connected with the cavernous sinuses (a) a 

 vein which traverses the foramen ovale, or the foramen Vesalii, 

 and connects the cavernous sinus with the plexus of veins 

 around the external .pterygoid muscle ; (b) a plexus of veins 

 which passes through -the temporal bone with the internal 

 carotid artery and connects the cavernous sinus with the 

 pharyngeal venous plexus ; (c] in a sense the ophthalmic 

 vein may be considered an emissary vein, for although under 

 ordinary cfrcumstances it is a tributary of the sinus, blood can 

 flow through it in the opposite direction from the sinus into 

 the orbit, and then along the tributaries which connect the 

 ophthalmic vein with the angular vein, and along the channels 

 which connect the ophthalmic vein through the inferior 

 orbital fissure with the veins in the infratemporal region. 



