128 PHYSIOLOGY. 



"Lastly, I mention 'the increased impetus of the blood 

 in the vessels of the brain.'' This I take to be the principal 

 cause by which the active state of our system is kept up, and 

 therefore by which the waking state of it is especially sup- 

 ported, and on many occasions produced. There is not a 

 matter of more common experience in physic. No man can 

 sleep while he can take notice of the throbbing or pulsation 

 of the arteries of his temples; and when in fevers there is a 

 constant state of waking induced, in nine * cases out of ten it 

 is accompanied with an increased impetus of the blood in the 

 vessels of the brain. We know also that sleep is incompatible 

 with the phrenitic state ; and the most remarkable instances of 

 that impetus is, in this and some other cases, from a resistance 

 of the blood in the lower extremities, whence it is determined 

 to the brain, so that nobody can sleep with cold feet. Among 

 the causes inducing a morbid vigilium is enumerated the ' rap- 

 tus humorum ad superiora? 



" To the action of the brain, as I have said, a certain degree 

 of tension in its blood-vessels is necessary. In the case of deli- 

 quium animi which follows blood-letting, it has been supposed 

 that a quantity of blood drawn away prevents the return of the 

 due quantity to the heart ; but considering the small quantity 

 compared with that which should supply the heart, it is not 

 likely to be owing to this ; and we observe that the deliquium 

 does not happen while the ligature is still upon the arm, but 

 in nine cases out of ten, takes place after the ligature is un- 

 tied : the solution of which, therefore, is, that there is no deriv- 

 ation from the heart while the ligature was applied, for the 

 resistance to the venous blood operates as much in keeping all 

 the vessels full, as is sufficient to compensate the loss of the 

 blood drawn off; and it is only when the ligature is taken off, 

 that the blood can flow from the extreme arteries, in conse- 

 quence of which there is a derivation by the subclavian, and by 

 the carotid, and a subtraction of the quantity going to the brain. 

 In most cases we can, by putting the body into a horizontal 

 posture, effectually prevent that derivation from the vessels of 

 the head. This effect of the erect and horizontal posture, in 

 favouring or preventing this deliquium animi, shews that a cer- 

 tain fulness is necessary to the energy of the brain being exert- 



