CIRCULATION IN THE CRANIAIj iC^IfY. 101 



into the external jugular, and distinct pulsatiohs, s\ 

 ments of respiration, may be produced in this way. 



It is evident that there are various other conditions which may impede 

 the venous circulation. Accidental compression may temporarily arrest the 

 flow in any particular vein. When the whole volume of blood is materially 

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

 quantity of blood accumulates chiefly in the venous system and proportion- 

 ally diminishes the rapidity of the venous circulation. 



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

 be 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 con- 

 tain but little blood, and in the other the circulation 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 which are habitually 

 dependent. The influence of gravity is exemplified in the production of 

 varicose veins in the lower extremities. This disease is frequently produced 

 by occupations which require constant standing ; but the exercise of walking, 

 aiding the venous circulation, as it does, by the muscular effort, has no such 

 tendency. 



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, which have been 

 regarded as capable of essentially modifying the circulation. 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 widely different from those presented in other parts of 

 the system. The venous passages in the brain, which correspond to the great 

 veins of other parts, are in the form of sinuses between the folds of the dura 

 mater and are but slightly dilatable. In the perfectly consolidated adult head 

 the blood is not subjected to atmospheric pressure, as in other parts, and the 

 semi-solids and liquids which make up the encephalic mass can not increase 

 in size in congestion and diminish in anaemia. Notwithstanding these con- 

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

 death show great differences in the quantity of blood which they contain. 

 The question then arises as to what is displaced to make room for the blood 

 in congestion; and what supplies the place of the blood in anaemia. An ana- 

 tomical peculiarity which has not yet been considered offers an explanation of 

 these phenomena. Between the pia mater and the arachnoid of the brain and 

 spinal cord there exists a liquid, the cephalo-rachidian fluid, which is capable 

 of passing from the surface of the brain to the spinal canal and communicates 

 with the fluid in the ventricles (Magendie). The communication between the 

 cranial cavity and the spinal canal is very free. It is easy to see one of the 



