876 HUMAN ANATOMY. 



plexus (page 873) and the ophthalmic vein (page 879), all these making connections 

 with the sinuses, on the one hand, and with extracranial veins, on the other. It is 

 customary, however, to limit the term to certain veins which, tor the most part, 

 traverse special foramina in the cranial walls, a few, however, passing through 

 foramina whose principal content is one of the cranial nerves. 



1. The parietal emissary vein (emissarium parietale ), rather variable in size, traverses the cor- 

 respondingly variable parietal foramen, placing the superior longitudinal sinus in communication 

 with the \eins of the scalp, ' 



2. The occipital emissary vein (emissarium occipitale) traverses the occipital protuberance 

 and places the torcular Herophili or one or the other of the lateral sinuses in communication with 

 the occipital veins. Its size is variable ; it usually receives the occipital diploic vein, and may 

 perforate only the external or the internal table of the occipital bone, representing in such cases 

 the terminal portion of the diploic vein rather than a true emissary. 



3. The mastoid emissary vein (emissarium masloideum ) passes through the mastoid foramen 

 and places the lateral sinus in communication with either the occipital or the posterior auricular 

 veins. It is occasionally wanting, and, on the other hand, may be so large as to appear to be 

 the continuation of the lateral sinus, the terminal portion of that vessel between the mastoid and 

 jugular foramina being greatly reduced in size. 



4. The posterior condyloid emissary vein (emissarium condyioideum) is very inconstant, and 

 when present traverses the posterior condyloid foramen, extending between the lateral sinus near 

 its termination and the vertebral veins. 



5. The anterior condyloid emissary vein ( rete canalis hypoglossi) is a net-work which sur- 

 rounds the hypoglossal nerve in its course through the anterior condyloid foramen. From the 

 plexus two veins arise, one of which passes to the inferior petrosal sinus and the other to the 

 vertebral veins. 



6. The emissaries of the foramen ovale ( reie foraminis ovalis ) are formed by two veins which 

 communicate above with the cavernous sinus and pass to the foramen ovale, where they form a 

 plexus surrounding the mandibular division of the trigeminal nerve and communicate with the 

 pterygoid plexus of veins. Occasionally, also, a similar plexus accompanies the maxillary division 

 of the trigeminus through the foramen rotundum. 



7. The emissary vein of the foramen of Vesalius is, like the foramen, inconstant, occurring 

 only about once in three cases. It extends between the cavernous sinus and the pterygoid 

 plexus of veins. 



8. Finally, a variable number of small veins pass through the connective tissue which 

 closes the foramen lacerum medium and place the cavernous sinus in communication with the 

 pterygoid plexus. 



Practical Considerations. — The relations of the emissary veins explain many 

 cases of spread of extra-cranial infection to the meninges and the sinuses. If there 

 were no emissary veins, injuries and diseases of the scalp and skull would lose 

 half their seriousness (Treves). Infected wounds of the scalp, cellulitis or erysipelas 

 involving that structure, osteomyelitis, or necrosis of the cranial bones may through 

 the emissary veins result in serious intra- cranial disease. The largest of these veins 

 is usually the mastoid, the communication between the lateral sinus and the occipital 

 or posterior auricular vein {vide supra). This relation and the considerable quan- 

 tity of blood carried by the mastoid vein are thought to explain the supposed effect 

 of leeches or blisters applied behind the ear in cerebral hyperaemia or inflammation, 

 especially as nearly all the blood of the brain leaves it through the lateral sinuses. 

 They also explain the extensive oedema behind the ear and around the mastoid 

 region often seen in lateral sinus thrombosis. Pus has formed in the cerebellar 

 fossa outside of the sigmoid sinus, made its exit through the mastoid foramen and 

 appeared as an occipito-cervical abscess (Erichsen). The escape of pus by the 

 mastoid foramen indicates extradural pus in the cerebellar fossa about the sigmoid 

 groove, with the probability that sigmoid sinus thrombosis exists, especially if the 

 mastoid vein is itself thrombosed (Macewen). 



In suppurative sigmoid sinus disease the posterior condyloid vein may convey 

 infection to the cellular tissue in the upper part of the posterior cervical triangle, 

 causing abscess beneath the deep fascia ; or, as a result of cerebellar pachymen- 

 ingitis, there may be phlebitis of this vein, with marked tenderness in the same 

 region. The emissary veins are important agents in the equalization of intra-cranial 

 pressure. 



