ZOOLOGY. - 269 



culations of physiologists are erroneously exaggerated, and that very 

 much less air (or, what comes to the same thing, air of less purity) is 

 needed daring sleep. He first appeals to animals. The lion, bear, or 

 tiger retires into his lair to sleep, quitting the open air, and excluding 

 it as much as possible. The dog seeks his kennel or corner, curls him- 

 self up, and bunes his head beneath his paws or body. Even birds, 

 those aerial creatures, who perish so rapidly when confined under a 

 bell-glass, and therefore seem peculiarly dependent upon fresh air, 

 when sleep approaches, hide their heads under their wings, the beak 

 covered with the soft down. Hibernating animals, as is well known, 

 never pass into their long sleep but when sheltered from the air. All 

 this is very true, but what about man ? Acting upon instinct, man 

 imitates the animals ; upon science, he does the very reverse. The 

 schoolboy, if he is cold, or if he cannot sleep, hides his head under the 

 bedclothes much as the bird hides its head under a wing. The unen- 

 lightened man or woman carefully draws the curtains round the bed. 

 The enlightened physician or nurse tears those curtains down. Sol- 

 diers and travellers " camping out" are obliged to cover their heads if 

 they wish to sleep ; and railway travellers at night, although six or 

 eight may be in one carriage, always finish by closing the windows. 



These, and other facts of similar significance, require to be well con- 

 sidered. Tire suggestion of M. Delbruck does not, we confess, present 

 a very acceptable aspect to us. He supposes that since plants, during 

 the night, absorb the oxygen which they exhale during the day, " an- 

 alogy would lead to the conclusion that animals at night absorb some 

 of the carbonic acid which they exhale during the day." Analogy is a 

 treacherous guide ; and in the present case, a more comprehensive ac- 

 quaintance with the physiological facts would have recognized the im- 

 perfection of the analogy. It is true, that plants absorb oxygen during 

 the night ; but it is only the woody parts, and these absorb it also during 

 the day, although the quantities are so small as to be almost inappreci- 

 able. It is the green parts which absorb carbonic acid during the day, 

 and these requiring the stimulus of sunlight, are inactive at night. 

 Nothing of the kind takes place in the animal organism. The^bl<st>d 

 refuses to absorb carbonic acid from the atmosphere, at all times, and 

 under all conditions. 



What, then, is our explanation of the paradox ? Why, if the fresh 

 air is so indispensable to the waking organism, is it less so to the sleep- 

 ing organism ? In other words, why can we sleep with a very moder- 

 ate supply of oxygen ? Physiology furnishes an answer. Sleep is a 

 condition during which the vital functions are all depressed. It, there- 

 fore, is incompatible with any excitement of the functions ; as we see 

 in the sleeplessness which succeeds over-fatigue or over-excitement, 

 (and which, by the way, suggests that the proverb, " After supper 

 walk a mile," must not be stretched to, "After supper walk five miles.") 

 Hence, the stimulating effect of oxygen too freely administered, is in- 

 stinctively avoided by man and beast, in order that sleep may be pla- 

 cid and undisturbed. Sleep requires diminished activity of the circu- 

 lation, and external warmth to compensate for this diminished activity. 

 Hence an atmosphere that is at once highly oxygenated and cold pre- 

 vents sleep. 



No rational reader will push these suggestions to absurd extremes : 

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