306 COSMIC rillLOSOMY. [ft. ti. 



once. Mr. Spencer adds that "the swallowing of food is 

 effected by a wave of constriction passing along the oeso- 

 phagus ; its digestion is accompanied by a muscular action 

 of the stomach that is also undulatory; and the peristaltic 

 motion of the intestines is of like nature. The blood 

 obtained from this food is propelled not in a uniform current 

 but in pulses ; and it is aerated by lungs that alternately 

 contract and expand." To this we may add that assimilation 

 is a continuous process of rhythmic interchange between the 

 molecular constituents of the various tissues and of the 

 blood by which they are bathed; that muscular action is the 

 result of a series of oscillatory movements; and that nervous 

 action depends upon a quickly alternating rise and fall in 

 the chemical instability of the molecules which compose the 

 nerve-centres. All these minor rhythms are as ripples upon 

 the surface of the longer rhythm constituted by sleep and 

 wakefulness. Eecent researches have shown that sleep itself 

 furnishes a beautiful illustration of the manner in which 

 rhythm is necessitated by the continual redistribution of 

 forces in the organism. According to the most recent view, 

 sleep is caused by a diminution in the capacity of the 

 cerebral arteries, which lessens the circulation of blood 

 through the brain. It is the sympathetic nerve which effects 

 this contraction of the arteries. During the day the activity 

 of the cerebrum itself supplies the stimulus which causes 

 arterial blood to flow through the head in large quantities, so 

 as to keep the vessels duly distended. But after many hours 

 of activity the ratio of repair to waste is sensibly diminished; 

 there is a fall in the average chemical instability of the 

 cerebral nerve-molecules, and a consequent diminution in the 

 amount of cerebral stimulus ; until presently the amount of 

 stimulus sent up from moment to moment along the cervical 

 branch of the sympathetic nerve exceeds the amount which 

 the cerebrum can oppose to it. Experiment has shown that 

 the effect of stimulating the sympathetic nerve is to contract 



