HYPEILEMIA OP THE BRAIN AND ITS MEMBRANES. 177 



collateral fluxion " to the brain, we may mention the habitual cere- 

 bral hyperaemia where there is contraction or closure of the aorta at 

 the point where the arterial duct terminates in the aorta (Volume I., 

 page 363). It frequently results from compression of the abdominal 

 aorta and its branches, by the distended intestines and by exudations. 

 In the same way obstruction of the cutaneous circulation during the 

 cold stage of intermittent fever, and from the action of severe cold, 

 induces collateral fluxion to the brain. According to Watson, in cold 

 nights many unfortunates are arrested in the streets for being drunk, 

 when they are only suffering from cerebral hyperaemia as a result of 

 disturbance of the cutaneous circulation. It is not improbable that 

 severe muscular exertion may also induce increased fulness of the 

 carotids and uxionary cerebral hyperaemia, by the pressure of the con- 

 tracted muscles on the capillaries. 



4. A fourth cause is paralysis of the vasomotor nerves of the cere- 

 bral vessels. Physiological experiments show that, if the cervical por- 

 tion of the 'sympathetic nerve be divided, the vessels on the correspond- 

 ing side of the head become dilated. The cerebral vessels appear to 

 be similarly affected by the use of spirituous liquors, by some poisons, 

 as well as by great emotions and excessive mental activity. I would 

 particularly call attention to the last cause, as I have frequently seen 

 dangerous .hyperaemia of the brain after too prolonged mental labor, 

 which resulted fatally from the occurrence of oedema. We can hardly 

 give any other explanation for these cases, than that the walls of the 

 vessels are paralyzed by the above influences, their calibre dilated, and 

 the supply of blood consequently increased. 



5. Lastly, fluxion to the brain results from atrophy of that organ. 

 The space left in the skull by the disappearance of the brain-substance 

 is partly filled by the dilatation of the vessels. We shall find this 

 dilatation of the vessels to be a frequent cause of their rupture ; and, 

 as atrophy of the brain often follows apoplexy, it is also a cause of 

 returns of apoplexy. We shall not attempt to say whether or not the 

 fluxionary hyperaemia of the brain, occasionally observed during con- 

 valescence from severe disease, is the result of atrophy of the sub- 

 stance of the brain or of the neuroglia and consequent dilatation of 

 the vessels. 



Passive hyperaemia, congestion of blood in the brain, depends 



1. On compression of the jugular veins and vena cava descendens. 

 As a type of this form, we may mention the excessive cerebral con 

 gestion caused 'by strangulation. The jugular veins are most fre 

 quently compressed by enlarged thyroid or lymphatic glands, the vena 

 cava descendens by aneurisms of the aorta. 



2. Congestion of the brain results from energetic expiratory move- 



