440 THE NERVOUS REGULATION OF THE BLOOD-VESSELS 



is compared with the total quantity of blood present in an animal of 

 this kind, it will be seen that the blood completes the circuit through 

 the hepatic blood-vessels once in every three minutes. But, while the 

 liver receives a larger supply of blood than any other structure in the 

 body, its vascularity per unit of substance is not so great as that of 

 the brain or kidney, because its weight is very much greater than that 

 of the organs just named. 



In agreement with its double blood supply, the liver is equipped 

 with a vasomotor mechanism which is capable of influencing the 

 arterial as well as the venous influx. By stimulation of the hepatic 

 plexus, as well as of single nerves thereof, it has been proved by Burton- 

 Opitz^ that these intrahepatic mechanisms are innervated by the 

 celiac ganglion. It has also been established that the aforesaid plexus 

 conducts afferent impulses from the liver, pancreas, and duodenum 

 to the solar division of the sympathetic system, whence they are trans- 

 ferred to the vagi and greater splanchnic nerves. 



D. THE CEREBRAL CIRCULATION 



The brain derives its blood from the internal carotid and vertebral 

 arteries, the anastomosis of the branches of these two systems at the 

 base of this organ being known as the circle of WilUs. This reservoir 

 serves to equalize the flow of blood to the various regions of the brain, 

 so that the obstruction of one or more of its tributary channels cannot 

 cause a complete anemia of this organ. Thus, if one carotid or one 

 vertebral is obHterated, an adequate supply of blood is nevertheless 

 obtained through the channels still left open. In fact, it has been 

 found that one vertebral is sufficient to furnish enough blood to retain 

 the brain in a functional condition. But, while the anastomosis is 

 complete between the blood-vessels situated at the base of this organ, 

 the distal or cortical vessels do not communicate very freely with one 

 another; indeed, several of them are terminal in their character. 



The cerebral veins, are classified in the same way, namely as 

 central or gangUonic and as distal or cortical. They do not, however, 

 descend in the path of the ascending arteries, but pursue in most cases 

 an independent course; in fact, some of them even ascend with the 

 iarteries. Besides, the blood stream in the smaller veins is frequently 

 opposite in direction to that in the larger collecting channel, so that 

 a certain impediment of the flow is produced at their points of con- 

 fluency. That this condition is physiological is evinced by the fact 

 that the lumen of the chief sinus is frequently rendered uneven by 

 trabeculae and that the orifices of its tributaries are guarded by valves. 

 Some sections of these collecting tubes may actually be placed in an 

 ascending position by moving the head. 



The venous sinuses of the cranial cavity of which there are eighteen, 

 are tubular blood spaces lined with endothelium and situated between 



1 Quart. Jour, of Exp. Physiol., xi, 1913, 57. 



