Emissary Veins 41 



All these veins have irregular communications with those of the 

 pericranium and dura mater, and, being by their nature incapable of 

 contraction, they are very prone to carry septic matter into the blood, 

 in the case, for instance, of compound fracture of the skull. By means 

 of the supra-orbital and neighbouring branches, the facial vein is in 

 direct communication with the ophthalmic vein, and so with the caver- 

 nous sinus. ' Thus may be explained the thrombosis of the sinuses 

 which sometimes follows facial erysipelas. 



Emissary veins are the short, open vessels which establish a de- 

 finite communication between the pericranial veins and the cerebral 

 sinuses. The mastoid emissary is a large link between the posterior 

 auricular or occipital vein and the lateral sinus, through the mastoid 

 foramen. The application of leeches behind the ear, as already 

 remarked, thus distinctly influences the intracranial circulation. 



The parietal emissary passes through the parietal foramen, between 

 a pericranial vein and the superior longitudinal sinus. A small condylar 

 emissary runs from an occipital vein through the posterior condylar 

 foramen into the lateral sinus, and a short vessel ascends from the 

 pterygoid plexus to the cavernous sinus. 



These emissary veins play an important part in septic wounds of 

 the scalp, rapidly carrying septic material into the intracranial sinuses, 

 and determining the onset of pyaemia. 



The cerebral veins, which lie in the sulci, are thin-walled and 

 valveless, and are in communication with each other across the 

 middle line through the medium of the sinuses. The superior set of 

 them open from behind forwards into the superior longitudinal sinus, 

 the lower ones end in the cavernous, petrosal, and lateral sinuses. 



The veins of the corpus striatum and of the choroid plexus emerge 

 from the velum interpositum, and, under the name of Galen's veins, 

 enter the straight sinus. 



The cerebellar veins open into the straight, the lateral, and the 

 petrosal sinuses. 



The pressure of venous blood within the skull is equalised by the 

 communication between the two cavernous sinuses by means of the 

 small circular sinus around the pituitary body ; by the transverse sinus 

 which runs across the basilar process ; by the communication between 

 the lateral sinuses at the torcular, and by the thin-walled veins upon 

 the surface of the brain which, destitute of valves, lie in the sulci and 

 communicate freely in all directions. 



The arteries of the dura are anterior meningeal from the 

 ethmoidal of the ophthalmic, and others from the internal carotid ; 

 middle meningeal from the internal maxillary, entering by the foramen 

 spinosum, the small meningeal entering by the foramen ovale ; and 

 a twig or two from the ascending pharyngeal, through the middle 

 lacerated foramen. Posterior meningeal come from the vertebral and 

 from the occipital through the posterior lacerated foramen, and perhaps 



