ch. ii.] RHYTHM 307 



the muscular walls of the cerebral arteries. The supply of 

 arterial blood is thus so far diminished that consciousness 

 ceases. But now the other half of the rhythm begins. The 

 cessation of conscious activity greatly diminishes the waste 

 of cerebral tissue ; and, although repair is also somewhat 

 lessened by the lessened blood-supply, yet the ratio of repair 

 to waste is increased. The complex nerve-molecules are 

 built up to higher and higher grades of instability, until it 

 only needs a slight stimulus from without, in the shape of a 

 sensation of sound or of light or of touch, to elicit a discharge 

 of nerve-force from the cerebral ganglia. This discharge is 

 instantly answered by a rush of blood, which distends the 

 cerebral arteries, revives consciousness, and holds in abeyance 

 the contractile energy of the sympathetic nerve, until the 

 decreasing ratio of repair to waste by and by necessitates a 

 recurrence of the rhythm. Thus the alternation of sleep 

 and wakefulness is due to a periodic variation in the ratio 

 between the amount of nerve-force stored up in the cerebrum 

 and the amount stored up in the sympathetic ganglia. We 

 recognize this truth in practice when we seek to induce sleep 

 by stimulating the sympathetic nerve with such substances 

 as bromide of potassium. 



The phenomenon of sleep is still further interesting as 

 the most familiar instance of the dependence of biologic 

 rhythms upon astronomic rhythms. All organisms, animal 

 and vegetable, from the highest to the lowest, exhibit alterna- 

 tions in the total distributions of their forces, which coincide 

 with the periodic appearance and disappearance of sunlight. 

 The longer astronomic rhythm, known as the earth's annual 

 revolution, causes corresponding rhythms in vegetable and 

 animal life; witness the blossoming and leafing of plants 

 in the spring, the revival of insec* activity at the same 

 season, the periodic flights of migratory birds, the hyber- 

 nating sleep of many vertebrates, and the thickened coats or 

 the altered habits of others that do not hybernate. If we 



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