MUSCLE 257 



severe shiver, termed a " rigor," is the result. At the same 

 time loss of heat by evaporation is checked, just as it is when 

 the skin is cold. The sweat-glands are rendered inactive. 

 A phenomenon which marks the nightly fall of temperature in 

 consumptive patients is the sudden return of activity in these 

 glands. 



Muscle when most highly developed has an extraordinarily 

 definite structure. It is minutely subdivided into units 

 which appear, looked at separately, simple in design. We 

 are tempted to believe that the explanation of the way in which 

 each of these units works is not far to seek. It is disappointing 

 to be obliged to admit that, notwithstanding all the thought 

 which has been devoted to the problem, we are as far as ever 

 from a definitive solution. We understand the principles on 

 which steam-engines, combustion-engines, electric motors are 

 planned. We compare muscle with each of these mechanical 

 contrivances in turn, expecting to discover the principle of its 

 construction. Many ingenious hypotheses have been formu- 

 lated ; but the fact that some of these are mutually destructive 

 shows clearly enough that as yet no approach to certainty has 

 been made. Probably the fundamental error lies in attempt- 

 ing to compare muscle with a mechanical contrivance. The 

 apparent simplicity and regularity of structure of " striped 

 muscle " misleads us. We ought to have commenced our in- 

 vestigations at the other end of the scale of mobile tissue to 

 have begun with semifluid and apparently homogeneous animal 

 matter, working upwards to the tissue which, being limited 

 to the one function of movement, and movement in one 

 direction only, has, as it were, crystallized along the lines of 

 force. 



All protoplasm is mobile. Its particles move one on another. 

 Hence follows either circulation of the living matter within the 

 cell or change in shape of the cell. The two phenomena are 

 identical in nature. Circulation is best studied in a large- 

 celled, transparent part of a plant. A filamentous water- weed 

 is suitable for the purpose. If this be examined with a micro- 

 scope while still alive, its cells are seen to contain a watery 

 juice enclosed in spaces of denser cell-substance. Bridges of 

 cell-substance span the spaces. The particles of which these 

 bridges consist are in a state of constant streaming motion, 



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