COMATA. S35 



effused may be of two kinds ; that is, they may be either a por- 

 tion of the common mass of blood, poured out from red vessels, 

 or a portion of serum or colourless fluid, poured out chiefly by 

 exhalents. 



MCIV. Of these several causes of compression, the first is 

 not to be considered here, because the removing it does not be- 

 long to our province ; and the consideration of the second may 

 be omitted, as in most instances it is neither to be discerned nor 

 cured by any means yet known. The third and fourth causes 

 of compression, as they are the most frequent, and are also most 

 properly the subjects of our art, so they are those which deserve 

 our particular attention ; and we shall therefore endeavour to 

 trace them further back in the series of causes which may pro- 

 duce them. 



MC V. Both the states of over-distention and of effusion may 

 be produced by whatever increases the afflux and impetus of the 

 blood in the arteries of the head ; such as violent exercise, a vio- 

 lent fit of anger, external heat applied, or any strong pressure 

 upon the descending aorta. 



MCVI. But both these states of over-distention and of effu- 

 sion, may also, and seem to be more frequently produced by 

 causes that operate by preventing the free return of the venous 

 blood .from the vessels of the head to the right ventricle of the 

 heart. 



MCVII. The venous vessels of the brain are of a conforma- 

 tion and distribution so peculiar, as to lead us to believe, that 

 nature intended to retard the motion of the blood, and accu- 

 mulate it in these vessels ; and, therefore, even very small addi- 

 tional resistances to the motion of the blood from these toward 

 the right ventricle of the heart, may still more readily accumu- 

 late the blood in them. Such accumulation will most readily 

 happen in advanced life, when the venous system in general is 

 in a plethoric state, and when this plethora takes place especially 

 in the venous vessels of the brain. It will, in like manner, be 

 most apt to occur in persons whose heads are large with respect 

 to the rest of the body ; and in persons of a short neck, which 

 is unfavourable to the return of the venous blood from the head. 

 The accumulation of blood in the venous vessels of the brain 



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