22 OF VITAL MOTION. 



be any periodical changes corresponding to those wit- 

 nessed in the milk-vessels ; but, at the same time, 

 there is a fact which may be construed to indicate the 

 existence of obscure changes in some measure ana- 

 logous. The concentric layers which occupy the in- 

 terior of many cells would seem indeed to refer to 

 periodical alterations, which may be understood to be 

 possible enough. We may suppose, indeed, a cell to 

 submit to a state of contraction after having attained 

 to its maximum state of dilatation, partly because its 

 contents have been abstracted by neighbouring cells 

 which are in process of development; and partly 

 because the same contents are less fitted to the gene- 

 ration of heat after having subserved in some measure 

 to the nutritive Avants of the part. And further, we 

 may suppose a second state of expansion during a 

 period of comparative inactivity in neighbouring 

 parts, which itself might result in contraction when 

 the contents were wasted by nutrition, or wanted for 

 the growth of other cells. 



Under similar circumstances, also, the same pro- 

 cesses might be repeated a number of times. In each 

 case we may suppose the formation of one of the con- 

 centric layers, which line the interior of the cavity, to 

 mark the epoch of nutrition, activity, and dilatation ; 

 and in this way the layers may be regarded as relics 

 which give evidence of slow periodical changes in these 

 cells, analogous, in some degree, to the more realized 



