488 NEUROLOGY. 



third ventricle, and the under surface of the cerebrum. Some- 

 times, instead of forming a single trunk, the anterior cerebral 

 arteries are joined by a transverse branch, termed the anterior 

 communicating artery. These vessels, or they and their com- 

 municating branch, form the antero-lateral and anterior boundaries 

 of the circle of Willis. 



The MIDDLE CEREBRAL ARTERY passes directly outwards to the 

 fissure of Sylvius, where it divides into numerous branches, which 

 ramify over the lateral and inferior surfaces of the cerebrum, and 

 anastomose with branches of the anterior and posterior cerebrals. 

 Just before this artery divides, it gives off a small branch, the 

 anterior choroid, which enters the descending cornu of the lateral 

 ventricle, and supplies the choroid plexus. 



VENOUS CIRCULATION. 



The encephalic venous circulation consists of veins and vascular 

 triangular channels of various sizes, found within the dura mater. 

 These are the venous sinuses, which are lined by a continuation 

 of the inner tunic of the veins, and which, like the other veins of 

 the brain, are destitute of valves, but present numerous bands or 

 cords stretched across their anterior so as to prevent distention. 

 The veins are divided into superficial and deep. 



The SUPERFICIAL VEINS are distributed over the whole surface, 

 mostly in the sulci, between the convolutions ; they receive small 

 capillary branches from the substance, and terminate, those from 

 the superior surface in the longitudinal sinus, and those on the 

 inferior and lateral surfaces in the lateral and other sinuses. 



The cerebellar veins are, like the cerebral, scattered over the 

 whole surface of the cerebellum, and mostly terminate in the 

 petrosal sinus. 



The DEEP or VENTRICULAR VEINS arise by very minute branches, 

 which form intimate plexuses in the ventricles, the chief being the 

 veins of the corpora striata, and those of the choroid plexuses, 

 uniting to form the venae Galeni, which leave the interior through 

 foramina at the posterior part of the longitudinal fissure, and ter- 

 minate in the longitudinal sinus. 



SINUSES. 



The principal sinuses of the dura mater are the superior and 

 inferior longitudinal, two lateral, two petrosal, two cavernous, the 

 occipital, and the transverse. 



