OP SLEEP. 203 



flu'ough it,* disposes to sleep.f But although this be granted, 

 It must be viewed not as the ordinary cause, but as a circum- 

 stance, or in fact a consequence, of sleep. Increase the activity 

 of an organ, you increase its circulation ; diminish its activity, 

 you diminish its circulation. The alteration of circulation is 

 usually not the cause, but the consequence ; necessary indeed to 

 the continuance of the altered degree of activity in the organ, but 

 not the cause. The degree of activity of any part, and the degree 

 of its circulation, are exactly and unalterably correspondent. If 

 the circulation through a part be mechanically increased or dimi- 

 nished, the sensibility and activity of the part will, doubtless, be 

 proportionally increased or diminished. This example occurs in 

 hemorrhage ; frequently both are affected simultaneously, when 

 diarrhoea renders the surface pale and cold, both the blood is sent 

 more sparingly to it, and the energy of its vessels is diminished 

 by the increase of energy in those of the intestines (Sect. VI. 

 Note.) But in ordinary sleep, the diminished circulation appears 

 only the consequence, for activity is always followed by inactivity. 

 Stimulate a muscle, separated from the body, it contracts, but it 

 soon refuses to do so ; after a little rest, it again contracts upon 



* As arterial blood when at rest acquires the venous character, it is evident 

 that in congestion of blood, by which is meant simply an unusual quantity of 

 blood in a part, not flowing with its usual freedom, the part affected has not its 

 proper supply of arterial blood. Hence congestion in the head must, from 

 this cause alone, produce drowsiness. 



t The phenomena of torpid animals are precisely analogous to those of com- 

 mon sleep. The sensibility and all the functions are lessened, the temperature 

 is low, the circulation slow, respiration almost or quite imperceptible, and 

 digestion suspended. This torpidity is produced by a deficiency of external 

 excitants, usually by cold and want of food, and, in the language of Brown, is 

 a state of direct debility, while our ordinary sleep is one of indirect clebi ity. 

 No structural peculiarity is discoverable, which enables certain animals to exist 

 in the torpid state. See Dr. Reeve's Essay on tite torpidity of animals, &c. 



Some animals become torpid on being deprived of moisture. A common 

 garden snail falls torpid if put in a dry place, and may be revived at any time by 

 the application of a little water. Moisture has revived some animalcules after 

 a torpidity of twenty-seven years. Spollanzaui, Opuscoli di Fiiica animate e 



