106 CIRCULATION OF THE BLOOD. 



attracted much attention among physiologists since the discovery of the circulation has 

 made it evident how important they may be in protecting the brain from reflux of 

 blood. When the act of expiration arrests the onward flow in the veins near the thorax, 

 these valves are closed and effectually protect the brain from congestion by regurgita- 

 tion. The blood accumulates behind the valves, but the free communication of the inter- 

 nal jugular with the other veins of the neck relieves the brain from congestion, unless 

 the effort be extraordinarily violent and prolonged. 



The above remarks with regard to the influence of expiration are applicable to vocal 

 efforts, violent coughing or sneezing, or any unusual muscular efforts, such as straining, in 

 which the glottis is closed. 



Eegurgitant Venom Pulse. In the inferior animals, like the dog, if the external 

 jugular be exposed, a distention of the vessel is seen to accompany each expiratory act. 

 This is sometimes observed in the human subject, when respiration is exaggerated, and 

 has been called improperly the venous pulse. There is no sufficient obstacle to the regur- 

 gitation of blood from the thorax into the external jugular, and distinct pulsations, syn- 

 chronous with the movements of respiration, may be produced in this way. 



It is evident that there are various other circumstances 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 proportionately 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 mount 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 contain 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 

 apd the valves are most numerous 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 induced 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 pecu- 

 liarities in the anatomy of some of the vessels, with exceptional conditions of the blood 

 as regards atmospheric pressure, which have been considered capable of essentially modi- 

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

 incompressible cerebral substance, and blood ; conditions which are widely different from 

 those presented in other parts of the system. On this account some have gone so far as 

 to consider that any change in the quantity of circulating fluid in the brain is a physical 

 impossibility. Pathological facts in opposition to such a view are so numerous and well 

 established that the question does not now demand extended discussion; but it is never- 

 theless true that there are anatomical peculiarities in these parts, the effects of which 

 on the circulation present important and interesting points for study. 



In the brain, the venous passages 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 semisolids and liquids which com- 

 pose the encephalic mass cannot increase in size in congestion and diminish in anae- 

 mia. Notwithstanding these conditions, the fact remains, that examinations of the 

 vessels of the brain after death show great differences in the quantity of blood which 



