176 DISEASES OF THE-BRAIX. 



textus cellulosus subaraclmoidealis contain a greater amount of serum. 

 Only when the brain is atrophied do we find oedema of the membranes 

 with overfilling of the vessels ; and only when an effusion of blood, a 

 tumor, or a collection of fluid in the ventricles, has contracted the space 

 in the skull do we find, along with anaemia, dryness of the membranes, 

 and disappearance of the sulci between the cerebral convolutions. 



The division of cerebral hyperaemia into active and passive, or, to 

 retain the expressions previously used, into fluxionary and congestive, 

 is practically valuable not only on account of the consideration being 

 easier, but because the symptoms of one form differ from those of 

 another. 



Fluxionary hyperaemia results 1. From" increased heart-action. 

 It is true that, in this case, while the arteries are fuller than usual, the 

 veins are less so ; hence the entire amount of blood in the vessels of 

 the brain and its membranes is not increased by the stronger action 

 of the heart. But the increased lateral pressure induces increased ful- 

 ness of the capillaries, and it depends chiefly on these (not on the 

 amount of blood in the large vessels) whether or not the brain acts 

 normally and is normally nourished. This form of cerebral hyper- 

 aemia occurs temporarily from augmented energy of contraction of a 

 healthy heart, as in fever and great bodily or mental excitement ; it is 

 habitual in the permanently increased activity of a hypertrophied 

 heart, but only when the hypertrophy is an independent disease, or in 

 case it accompanies an obstruction to circulation, when it has become 

 greater than is necessary for the compensation. Simple, non-com- 

 plicated hypertrophy of the heart is not frequent, and occurs almost 

 exclusively in topers and persons who continually do hard work. On 

 the other hand, hypertrophy that has become greater than was neces- 

 sary to compensate the obstruction to the circulation is quite frequent. 

 Examples of this are the occasional enormous hypertrophies of the left 

 ventricle when there is insufficience of the aortic valves, and perhaps 

 also the hypertrophy of the heart in morbus Brightii. 



2. Fluxionary hyperaemia of the brain results from too slight resist- 

 ant power of the afferent blood-vessels, whether this be congenital or 

 acquired. When the cerebral arteries have delicate, thin walls, so 

 that they yield to an increased pressure of the blood sooner than the 

 other arteries of the body do, and hence, when the action of the heart 

 is only moderately increased, fluxionary hyperaemia of the brain is in- 

 duced, it is customary to say that the person so affected has a tendenc} 

 to " rush of blood to the head." 



3. Fluxionary hyperaemia to the brain results from an increase of 

 the lateral pressure in the carotids as a consequence of obstructed 

 escape of blood from the aorta into other branches. As a type of this 



