Conjectures on the proximate Cause of Sleep. 263 



The nervous fluid and animal spirits have long since been ex- 

 luded from all agency in the system. It is, therefore, scarcely 

 necessary to advert to the antiquated theory of Haller, who, in 

 seeking for the proximate cause of sleep, conjectures that this 

 phsenomenon " arises either from a simple absence, deficiency, 

 and immobility of the spirits, or from compression of the 

 nerves, and always from the motion of the spirits through the 

 brain being impeded *." " But that, if while the rest of the 

 emporium of the senses and muscular motion is at rest, some part 

 remains open, is pervaded by the spirits, and watches," then, 

 that our dreams occur ; and also somnambulism, " if certain vo- 

 luntary motions are conjoined with the perceptions of the mindf." 

 Yet, it is satisfactory to perceive, that all the explanation that 

 this great physiologist endeavoured to derive from these imagi- 

 nary essences, to satisfy his rational thirst of inquiry, may be 

 found in the substantial reality of the brain and nerves — their 

 partial exhaustion by exercise, and their indispensable renewal 

 by the process of assimilation. 



This diurnal operation may begin later, or cease earlier, in some 

 portions of the brain and nerves than in others. Those portions, 

 while exempt from its influence, may be as active as the entire 

 system would be, were the individual awake. The thoughts 

 which originate in these vigilant organs, not being compared by 

 means of the senses with external objects, assume the substantial 

 forms of reality, and constitute our dreams. Volition, as far as 

 it is inherent in any organ, may exert itself to the extent of its 

 power. But it can have no power to stimulate the neighbouring 

 organs which continue asleep, or to put the limbs in motion, 

 whose nerves remain subjected to the assimilating process which 

 renders them torpid. But if those nerves have recovered from 

 its effects, they will naturally submit to any volition accustomed 

 to govern them — and this circumstance will account for all the 

 perplexing mysteries of somnambulism. This phaenomenon is 

 of rare occurrence ; and the nerves of motion are so seldom ex- 

 erted in sleep, that hitherto the will has then been supposed in a 

 state of abeyance. But we can much more rationally account 

 for the various phenomena of dreams, night-mare and somnam- 

 bulism, by supposing that the will may be active in any of the 

 cerebral organs which happen to be awake, yet destitute of power 

 to put the limbs in motion as long as the nerves of those limbs 

 are involved in the stupor of sleep, and invested with this power 

 from the moment that the stupor in cjuestion is removed from 

 those nerves. But it is not to be forgotten, that if the brain be 

 altogether paralysed by sleep, so must the whole body. Som- 



• Haller's First Lines of Physiology, p. 285. Edinburgh, 1801. 

 t Id. p. 283. 



R 4 nambulism, 



