i88 THE RIVER-SIDE NATURALIST. 



stituent matter of those structures. The duration of the 

 periodical state of sleep, and the time at which it occurs, 

 are dependent partially on external and partially on internal 

 causes. 



"The causes determining duration of sleep and waking 

 are seated, however, in the organism itself. The periodi- 

 cal recurrence of sleep and the waking state is, therefore, 

 essentially connected with something in the nature of 

 animals, and is not dependent on the simple alternations of 

 day and night. The sleep of animals is a phenomenon 

 dependent on a change in the animal part of the organism 

 alone. All the functions of organic life namely, the pro- 

 cesses ministering to nutrition, with all the involuntary 

 movements attending them pursue their ordinary course. 

 Even the involuntary movements of the animal system of 

 muscles, such as those of respiration, and many other 

 movements of the same kind, do not partake of the repose 

 of sleep. The organic system has its period of remission 

 and rest, but these are not coincident with the sleep of 

 animal life, and are very different for different organs. 



"All the phenomena of organic life, and indeed all the 

 phenomena presented by the animal body, with the excep- 

 tion of the true animal functions which are under the 

 influence of the mind, obey a law of absolute necessity, and 

 the nutrition and maintenance even of the organs of animal 

 life are not dependent on the operations of the mind or 

 intellect. 



"We may, therefore, regard sleep and the waking state 

 as the result of a species of antagonism between the organic 

 and the animal life, in which the animal functions, governed 

 by the mind, from time to time become free to act, while at 

 other times they are repressed by the organic force acting 

 in obedience to a law of creative nature. 



" Sleep, in a greater or less degree, as Aristotle correctly 

 remarked, falls to the share of all animals. Some even 

 dream the dog, for instance, barks in his sleep. In some 

 the periods of sleep are less distinct and regular ; and this 

 is particularly the case in the cold-blooded animals. They, 

 however, appear to be subject to states analogous to sleep. 



