THE BLOOD- VASCULAR SYSTEM. 



in the cranial wall and establish communications between the sinuses inside the 

 skull and the veins external to it. Some of these are always present, others only 



occasionally so. They vary much in size 

 in different individuals. The principal 

 emissary veins are the Yollowing: 1. A 

 vein, almost always present, which passes 

 through the mastoid foramen and con- 

 nects the lateral sinus with the posterior 

 auricular or with an occipital vein. 2. A 

 vein which passes through the parietal 

 foramen and connects the superior longi- 

 tudinal sinus with the veins of the scalp. 

 3. A plexus of minute veins which pass 

 through the anterior condyloid foramen 

 and connect the occipital sinus with the 

 vertebral vein and deep veins of the 

 neck. 4. An inconstant vein which 

 passes through the posterior condyloid 

 foramen and connects the lateral sinus 

 with the deep veins of the neck. 5. One 

 or two veins of considerable size which 

 pass through the foramen ovale and con- 

 nect the cavernous sinus with the ptery- 

 goid and pharyngeal plexuses. 6. Two 

 or three small veins which pass through 

 the foramen lacerum medium and connect 

 the cavernous sinus with the pterygoid 

 and pharyngeal plexuses. 7. There is 

 sometimes a small vein passing through 

 the foramen of A'esalius connecting the 

 same parts. K. A plexus of veins pass- 

 ing through the carotid canal and con- 

 necting the cavernous sinus with the in- 

 ternal jugular vein. 



Surgical Anatomy. These emissary 

 veins are of great importance in surgery. In 

 addition to them there are, however, other com- 

 munications between the intra- and extra-cra- 

 nial circulation, as, for instance, the communi- 

 cation of the angular and supra-orbital veins 

 with the ophthalmic vein at the inner angle of 

 the orbit (page 595), and the communication 

 of the veins of the scalp with the diploic veins 

 (page 599). Through these communications 

 inflammatory processes commencing on the out- 

 side of the skull may travel inward, leading to 

 osteo-phlebitis of the diploe and inflammation 

 of the membranes of the brain. To this in 

 former days was to be attributed one of the 

 principal dangers of scalp wounds and other 

 injuries of the scalp. 



By means of these emissary veins blood 

 may be abstracted almost directly from the intra- 

 cranial circulation. For instance, leeches applied 

 behind the ear abstract blood almost directly 

 from the lateral sinus by means of the vein 

 passing through the mastoid foramen. Again, 

 epistaxis in children will frequently relieve severe 



Fn;. 3SO. The superficial veins of the upper extremity, headache, the lilood which flows from the nose 



being derived from the longitudinal sinus by 



means of the vein which passes through the foramen caecum, which is another communication 

 1 iet ween the intracranial and extracranial circulation which is constantly found in children. 



