Conjectures on the proximate Cause of Sleep. 329 



laxation and rest ; neither lasts long ; the alteration is most ra- 

 pid : — yet if the designs of Providence require that assimilation 

 should in general take place during an interval of rest, the pre- 

 sent instance affords no exception ; and we may as readily con- 

 ceive that the coronary vessels may repeatedly pour out the 

 nervous secretion, at the moment the nerves remit their action, 

 as that the condensing valve of the steam-engine should permit 

 the escape of a due portion of water, at the requisite juncture. 

 Thus the phsenomena of organic life will present no anomaly in 

 the arrangements of nature ; and the law which governs these 

 circumstances, will be found not less general than any other of 

 the laws established by the Creator. 



It has next been objected, that the effects of the process of 

 assimilation on the brain and nervous system, cannot be consi- 

 dered as the cause of sleep, inasmuch as sleep is incident to plants 

 and many animals which are altogether destitute of brain and 

 nerves. With respect to plants, the phasnomenon is manifested 

 by the drooping or folding together of their leaves or leaflets ; and 

 this change is said to be occasioned by the luithdrawins of the 

 stimulus of light, and is merely presumed to be a state of rest to 

 their vital functions*. But this circumstance, whatever may be 

 its nature, is not bv any means so general in the vegetable king- 

 dom, as sleep in the animal ; and therefore its proximate cause 

 cannot possibly be regarded as so indispensable an agent as the 

 • assimilating process, or whatever else is the proximate cause of 

 the latter: — but the vital functions of plants have so little resem- 

 blance to the vital functions of animals, and are so utterly dissi- 

 milar to those superior functions which depend on a brain and 

 nervous system, (which are exclusively concerned in the hypo- 

 thesis,) that no accurate analogy can well be instituted between 

 them — much less between the conjectural rest of the one, and 

 the unequivocal repose of the other ; — and the naturalist, who 

 would seriously attempt to establish the comparison, might as 

 rationally pretend to ascribe both e'ffects to one and the same 

 cause, and decide that the sleep of animals, as well as plants, is 

 occasioned by " the withdrawing of the stimulus of light." 



But with respect to those inferior creatures which arc destitute 

 of brain and nerves, it is maintained by physiologists, that the 

 nerTous substance is irregularly diffused through the entire of 

 their structure : there is therefore no necessity to seek for a pe- 

 culiar explanation of their sleep. Whatever be their mode of 

 imbiliing nourislnnent, they must necessarily be in a state of 

 wakefulness while employed in the act ; but after they have di- 

 gested their food, it must, as in superior animals, be conveyed 

 through their system, however little analogous the instruments of 

 • Sec Rtcs's C'yc%aY/w, article si.kf.p or vlants. 



transmission. 



