878 HUMAN ANATOMY. 



crus is lacking, the vein then emptying into the cavernous sinus. The deep Sylvian vein, which 

 is its main stem of origin, begins in a number of vessels which ramify over the surface of the 

 insula (island of Reil) and passes downward and forward at the bottom of the Sylvian fissure to 

 become continuous with the basilar at the anterior perforated space. Occasionally it unites 

 with the lower portion of the middle cerebral vein or opens with it into the spheno-parietal 

 sinus. The anterior vein of the corpus callosum corresponds to the anterior cerebellar artery, 

 sometimes termed the anterior central vem ; it arises on the anterior part of the upper surface 

 of the corpus callosum and bends downward around the genu to unite with the deep Sylvian 

 vein at the anterior perforated space. 



The basilar vein drains all the central part of the base of the brain, and, in addition to the 

 two veins which are regarded as its stems of origin, it receives branches from the optic tract, 

 the olfactory bulb, the anterior perforated space, the tuber cinereum, the corpora mammillaria, 

 and the posterior perforated space, and it furthermore receives a vein from the superior vermis 

 of the cerebellum. The veins of the anterior perforated space are from ten to fifteen in number 

 and have their origin in the nuclei of the corpus striatum arid in the internal capsule, while those 

 of the posterior perforated space drain the optic thalami. 



Practical Considerations. — The free communication of the thin-walled 

 valveless cerebral veins with one another is one of the agents for the equalization of 

 intracranial venous pressure. An anastomotic trunk unites the middle cerebral vein 

 with the posterior cerebral, thus permitting the passage of venous blood by means 

 of the anterior basilar vein into the sinuses about the foramen magnum. Relief from 

 excessive intracranial blood-pressure may, in addition, be effected by the escape of 

 blood from within the cranium (a) in the occipital region through the internal 

 jugular and mastoid vein ; (d) in the frontal region through the ophthalmic vein 

 and the vein traversing the foramen ovale ; (c) in the basal region through the 

 petrosal sinuses and the posterior condyloid vein ; and (d) at the vertex through the 

 diploic veins and the venules penetrating the outer table of the cranium to join those 

 of the scalp (Allen). 



The avoidance of sudden depletion of the intracranial venous channels through 

 the inspiratory emptying of the large extracranial veins is admirably provided for 

 and the mechanism should be understood, as it has practical relation to many phe- 

 nomena of cerebral anaemia and hyperaemia, to shock and syncope and concussion, 

 to sinus thrombosis, and to many other intracranial conditions. The chief factors 

 in equalizing the flow in the sinuses — and thus practically throughout the brain — 

 may be briefly summarized as follows : 



(a) The oblique entrance into the longitudinal sinus of its tributaries — the 

 larger middle and posterior cerebral veins — pouring their blood into it against the 

 stream ; (d) the division of the sinus at the Torcular Herophili into two trunks 

 diverging at right angles ; (r) the course of the blood-current in the lateral sinus — 

 first horizontal, with a convexity outward ; then — in the first part of the sigmoid — 

 vertical ; then horizontal, with a convexity downward, and then a quick upward and 

 outward turn, with narrowing of its calibre before entering the jugular fossa ; (d) 

 the widening of the upper part of this fossa — which is above the outlet of the sig- 

 moid — and the narrowing of its exit (Macewen). Were it not for these and other 

 subsidiary anatomical .arrangements contributing to the same end, the effect of a 

 deep inspiration on the cervical veins (page 863) would be so to aspirate the venous 

 channels of the brain as to cause faintness or momentary unconsciousness. 



The cerebral veins are so delicate that in operations upon the brain it is often 

 better to arrest bleeding by gauze-pressure than to attempt to seize and tie separate 

 vessels. 



5. The Cerebellar Veins. — The cerebellar veins form a net-work over the 

 surface of the cerebellum, the course of the larger stems being, for the most part, at 

 right angles to that of the foliae. 



The superior cerebellar veins (vv. cerebelli superiores) open in part laterally 

 into the lateral and superior petrosal sinuses, while others pass medially and unite to 

 form a superior median cerebellar vein, which passes forward and downward along 

 the superior vermis and opens either into the great cerebral vein or the terminal 

 portion of the basilar vein. 



