CIRCULATION IN THE CRANIAL CAVITY 8 1 



venous circulation. Accidental compression may temporarily arrest 

 the flow in some one vein. When the volume of blood is materially 

 increased, as after a full meal with copious ingestion of liquids, the 

 additional quantity of blood accumulates mainly in the venous system 

 and proportionally diminishes the rapidity of the venous circulation. 



The force of gravity also has an important influence. It is more 

 difficult for the blood to pass from below upward to the heart than to 

 flow downward from the head and neck. The action of this is seen if 

 comparison is made between the circulation in the arm elevated above 

 the head and hanging by the side. In the one case the veins are 

 readily emptied and contain but little blood, and in the other, the circu- 

 lation is more difficult and the vessels are moderately distended. The 

 walls of the veins are thickest and the valves are most abundant in 

 parts of the body that are habitually dependent. 



CIRCULATION IN SPECIAL PARTS 



Circulation in the Cranial Cavity. In the encephalic cavity there 

 are certain peculiarities in the anatomy of some of the vessels, with 

 exceptional conditions of the blood as regards atmospheric pressure, 

 that have been regarded as capable of considerably modifying the circu- 

 lation. In the adult the cranium is a closed air-tight box, containing 

 the incompressible cerebral substance, blood, lymph and the cephalo- 

 rachidian fluid ; and the blood is here under conditions quite different 

 from those presented in other parts. The venous passages in the brain, 

 which correspond to the great veins in other situations, are in the form 

 of sinuses between the layers of the dura mater and are but slightly dilat- 

 able. In the perfectly consolidated adult head, the blood is not sub- 

 jected to atmospheric pressure, as in other parts, and the semisolids 

 and liquids which make up the encephalic mass can not increase in size 

 in congestion and diminish in anemia. Nothwithstanding these con- 

 ditions, the fact remains that examinations of the vessels of the brain 

 after death show differences in the quantity of blood. The question 

 then arises as to what is displaced to make room for the blood in con- 

 gestion, and what supplies the place of the blood in anemia. An 

 anatomical peculiarity not yet considered offers an explanation of 

 these conditions. Between the pia mater and the arachnoid of the 

 brain and spinal cord there exists a liquid, the cephalo-rachidian fluid, 

 that may pass from the surface of the brain to the spinal canal and 

 communicates with the contents of the ventricles (Magendie). The 

 communication between the cranial cavity and the spinal canal is quite 

 free. It is easy to see one of the physiological uses of this liquid. 



