24 THE MENINGES OF THE BRAIN. 



cord; perivascular lymph spaces carry the fluid from the interior 

 to the subarachnoid spaces. 



B. THE CIRCULATION OF THE RHOMBENCEPHALON. 



Bl. The medulla oblongata is supplied with blood by the 

 following branches of the vertebral artery: The posterior and 

 the anterior spinal, the posterior inferior cerebellar and several 

 short bulbar arteries (Fig. 9). The posterior inferior cerebellar 

 (a. cerebelli inferior posterior} winds from before backward around 

 the medulla, runs between the vagus and accessory nerves, enters 

 the vallecula cerebelli and gives branches to the medulla and to 

 the chorioid tela of the fourth ventricle. The anterior spinal 

 artery (a. spinalis anterior} formed by the y-like union of a branch 

 from each vertebral artery, descends along the anterior median 

 fissure; and the posterior spinal artery (a. spinalis posterior] of 

 either side, rising from the vertebral near the lower end of the 

 medulla, descends in front of the posterior lateral sulcus. Both 

 distribute branches along their course. The branches for the 

 most part enter the median raphe or follow the roots of the bulbar 

 nerves, suggesting the centrifugal and centripetal arteries of the 

 spinal cord (Fig. 13). The veins pursue much the same course 

 as the arteries. The anterior median vein joins the ventral veins 

 of the pons and is drained into the cerebellar veins or directly 

 into the superior petrosal sinus. The posterior median vein bifur- 

 cates y-like at the middle of the medulla and the two branches 

 wind around the medulla to its anterior surface and empty into 

 the inferior petrosal sinus or the basilar plexus. Issuing from 

 the medulla with the roots of the ninth to the twelfth cerebral 

 nerves are three or four small veins, the radicular veins, which 

 run into the occipital and inferior petrosal sinuses (Cunningham). 

 Both arteries and veins possess perivascular lymph spaces, but 

 there are in the medulla no lymphatic vessels. 



B2. The pons Varolii is supplied by the pontal, the supe- 

 rior cerebellar and the posterior cerebral branches of the basilar 

 artery (Fig. 9). The short and transverse branches of the basilar 

 artery, the pontal arteries (aa. pontales), furnish the greater portion 

 of blood to the basilar area of the pons, while the superior cere- 



