130 OF THE STRUCTURAL ELEMENTS OF THE HUMAN BODY. 



still increasing in size, in the direction of least resistance; in the same manner 

 as the roots of plants send forth their prolongations, sometimes of extraordinary 

 length, through the soil which they can most readily traverse. But there is 

 reason to believe that such alterations of form may also be effected, even after 

 the cells have attained their full dimensions, by the agency of currents within 

 the cell, whose maintenance is intimately connected with its nutrient operations, 

 and whose " set" in particular directions determines the protrusion of the cell- 

 wall at the points towards which they tend. It is well known that such currents 

 are frequently to be observed in the cells of Plants ; indeed, it seems probable 

 that they take place in every vegetable cell at a certain stage of its development. 1 

 Regular currents, apparently of a similar nature, and quite distinct from the 

 " molecular movements" which cannot be justly attributed to any other than 

 physical causes ( 101), have also been witnessed within many Animal cells : 

 thus Prof. Sharpey has seen a clump of dark granular matter in regular revo- 

 lution, and numerous separated granules coursing round and round, within the 

 spheroidal pigment-cells of the tail of a Tadpole. 9 Prof. Czermak has described 

 peculiar rotatory movements in certain vesicles attached to the fine extremities 

 of the seminal tubes in the salamander. 3 Prof. Kb'lliker has observed analogous 

 movements in the cells of the seminal filaments of a Polyclinum, and in large 

 cells in the sprouting arms of a Medusoid animal ; 4 and a still more remarkable 

 case, in which the currents within the cell distinctly determined the protrusion 

 of the cell-wall in one direction or anotUer, has been witnessed by the Author 

 in one of the lowest forms of animal life, and has been elsewhere described by 

 him in detail. 5 On the whole, then, we seem entitled to affirm that such 

 permanent alterations in the forms of cells, as cannot be explained by external 

 pressure, are dependent upon an agency of the same kind with that which is 

 concerned in the ordinary operations of development; and must thus be re- 

 garded as a continual manifestation of what may be conveniently designated 

 by the term Cell- force? 



110. Certain kinds of cells, however, exhibit more rapid changes of form, 

 such as cannot be so directly attributed to their nutritive operations ; and these 

 may take place in such a manner, as to communicate motion to objects external 

 to them. Moreover, these changes may take place altogether spontaneously 

 (i. e., independently of any external stimulation), or they may only occur when 

 some exciting influence is communicated to the cell. Of both forms of move- 

 ment, we have numerous illustrations in the Vegetable kingdom; the rhythmical 

 undulations of the elongated cell-filaments of the Oscillatoriae, or the alternat- 

 ing flexures of the stalks of the lateral leaflets of the Hedysarum gyrans, being 

 examples of the former; whilst the closure of the fly-trap of the Dionsea, the 



1 See "Prin. of Phys. Gen. and Comp.," \ 137, Am. Ed. 



2 Introduction to Dr. Quain's "Elements of Anatomy," 5th edit., p. 65, Am,. Ed., note. 



3 "Oester. Medic. Jahrbucher," Jan. 1845 ; and " Brit, and For. Med. Rev.," vol. xxii. 

 p. 264. 



4 " Entwick. der Cephalopoden," p. 156. 



s " Prin. of Phys. Gen. and Comp.," p. 244, Am. Ed. 



The Author is particularly desirous that he should be understood as implying by this 

 term, not that the force is produced or generated by the cell, but merely that the growth 

 of the cell is the most general manifestation of that force, to which it is convenient to refer 

 as a standard of comparison ; and also that the cell aifords the ordinary instrumental con- 

 dition for the exercise of the force in question, although it can doubtless be exerted in 

 many cases in which cell-development does not take place. The use which he would make 

 of the term is just that which is commonly made of the term " engine-power" in mechanics ; 

 for the steam-engine possesses no power in itself, but is simply the instrument most com- 

 monly employed, because the most convenient and advantageous yet devised, for the appli- 

 cation of the expansive force of steam, generated by the application of Heat, to the produc- 

 tion of mechanical motion ; this Heat, which is the real motive power, being capable of 

 manifesting itself in a great variety of other modes. 



