PHYSIOLOGY. 133 



many cases where there is a mixture of the excitement and col- 

 lapse; but that I take to be the state of waking, when the whole of 

 the functions of the brain can be exercised ; and, upon the con- 

 trary, I would call that more strictly a state of collapse, in which 

 these functions are for the most part suspended ; and so upwards 

 or downwards as it may occur." This excitement may be consi- 

 dered as of two kinds, either as it respects the vigour, or as it 

 respects the mobility of the system ; and these different states 

 of the brain are expressed in the body by strength or debility, 

 alacrity or sluggishness ; and, in the mind, by courage or timi- 

 dity, gaiety or sadness. "I formerly (LXXXIX.) took occa- 

 sion to distinguish between these two states of vigour and 

 mobility, the one expressing the force, the other the facility of 

 motion. I own that in particular instances you will find a cer- 

 tain degree of slowness joined with strength, and great alacrity 

 joined with debility : why that is so I cannot explain." 



CXXXIII. A degree of collapse takes place in the case of 

 natural sleep, when the collapse prevails so far as to suspend very 

 entirely the exercise of the animal functions; and, though the ex- 

 ercise of the vital and natural continue, they are considerably 

 weakened. " The fact has been disputed ; as some have alleged 

 that these functions are rather invigorated : but I think this 

 opinion is at present deserted ; and De Gorter, in his Exercita- 

 tiones de Somno et Vigilia, has proved that the vital and na- 

 tural functions go on more slowly and weakly in sleep. You 

 will find that Haller also has come to the same conclusion. 

 Why is the action of the vital functions only weakened and 

 not interrupted in ordinary sleep, as they are dependent upon the 

 brain ? Different theories have been proposed, such as the differ- 

 ence of origin of the nerves sent to the organs, of the vital, from 

 those of the animal functions, viz. that the latter are derived 

 from the brain, and the former from the cerebellum ; but at 

 present anatomists are agreed that there is no such distinction. 

 Haller thinks it enough to put it upon the force of habit, which 

 will go some length, but it does not completely explain the mat- 

 ter. I have formerly prepared for an explanation which is quite 

 easy : in the exercise of voluntary motion there is an in- 

 creased excitement, however momentary, in the action of the 

 brain, and a constant volition necessary, If, therefore, sleep 



