KINDS OF MOTION. 189 



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cles are only able to continue in exercise for a limited pe- 

 riod, during which, their irritability diminishes, and the fur- 

 ther exertion of their powers becomes painful. When thus 

 fatigued, animals endeavour to place themselves in a condi- 

 tion for resting, and fall into that stale of temporary lethar- 

 gy, denominated sleep. 



The positions assumed by animals during sleep, are ex- 

 tremely various. In the horse, they even differ according 

 to circumstances. In the field he lies down, in the stable 

 he stands. Dogs and cats form their bodies into a circle, 

 while birds place their heads under their wings. 



The ordinary time of sleep is likewise exceedingly vari- 

 ous in different animals, and in the same animal is greatly 

 influenced by habit. It in general depends on circumstan- 

 ces, connected with food. 



It is probable, that all animals, however low in the scale, 

 have their stated intervals of repose, although we are as yet 

 unacquainted either with the position which many of them 

 assume, or the periods during which they repose. 



There are many animals in which the muscular filaments 

 cease to be perceptible, as in many zoophytes ; yet, when 

 we see ah 1 the actions of the muscular fibres performed, such 

 as contraction and expansion, we admit their existence 

 from analogy, and repose the utmost confidence in our con- 

 clusions. 



In the classification of animals, the organs of motion are 

 very extensively employed. They, however, do not aid us 

 in forming divisions of the highest kind, but they assist in 

 the construction of subordinate groups. This may appear 

 obvious by an examination of the facts stated in this chap- 

 ter. Among Quadrupeds, for example, many walk, some 

 fly, others swim, among Reptiles, some possess feet, others 



