THE NERVOUS SYSTEM 1479 



cranial cavity through the antero-intemal compartment of the jugu- 

 lar foramen, to terminate in the bulb of the internal jugular vein. 

 It receives tributaries from the inferior surface of the cerebellum, 

 and from the internal ear. 



The transverse or basilar sinus is really a plexus of veins, situated 

 within the dura mater over the basilar process of the occipital bone, 

 and extending as low as the anterior margin of the foramen magnum, 

 where it communicates with the anterior intraspinal veins. It 

 connects the anterior ends of the inferior petrosal sinuses with each 

 other. 



The petro-squamous sinuses are two in number — right and left — 

 and are situated, on either side, along the junction of the petrous 

 and squamous parts of the temporal bone. The direction of each 

 is backwards, and it opens into the lateral sinus as that is entering 

 the sigmoid fossa of the pars mastoidea. 



Each petro-squamous sinus represents the continuation of the 

 lateral sinus, in early life, to terminate in the primitive jugular vein. 

 Sometimes there are no traces of these petro-squamous sinuses. 



Emissary Veins. — These axe vessels which pass through foramina in the 

 cranial wall, and serve to establish communications between the intracranial 

 venous sinuses and the extracranial veins. The principal emissary veins 

 are as follo^^-s : mastoid, nasal, posterior condylar, parietal, and occipital. In 

 addition to these there are emissary veins, which pass through (i) the foramen 

 ovale, {2) the foramen Vesalii (when present), and (3) the foramen lacerum 

 medium. 



The lateral sinus, in many cases, has two emissary veins, mzistoid and 

 posterior condylar. 



The mastoid emissary vein is of large size. It passes through the mastoid 

 foramen, and usually opens into the posterior jugular vein, but it may open 

 into the posterior auricular vein. 



The posterior condylar emissary vein passes through the posterior condylar 

 foramen, and opens into the suboccipital plexus, from which the blood is 

 carried away by the vertebral and deep cervical veins. 



The mastoid and posterior condylar veins are not constant. 



The superior longitudinal sinus hcis three emissary veins — nasal, parietal, 

 and occipital. 



The nasal emissary vein passes through the foramen caecum, in front of 

 the crista galli of the ethmoid bone, and it establishes a communication 

 between the superior longitudinal sinus and the veins of the roof of the nose. 



The parietal emissary vein passes through the parietal foramen, and opens 

 into the occipital plexus, or into radicles of the superficial temporal vein. 



The occipital emissary vein passes from the torcular Herophili to the occi- 

 pital plexus, being transmitted through a minute foramen which pierces the 

 external occipital protuberance. 



The nasal, parietal, and occipital emissai^r veins are inconstant. 



As previously stated, the cavernous sinus communicates with (i) the 

 angular vein of the face, (2) the pterygoid plexus, and (3) the pharj-ngeal 

 plexus. The marginal or inferior occipital sinus communicates with the pos- 

 terior intraspinal veins, and the transverse or basilar sinus communicates 

 with the anterior intraspinal veins. 



Arachnoid Membrane. 



This is a very delicate membrane which loosely surrounds the 

 encephalon, and is situated between the dura mater and pia mater. 



