PHYSIOLOGY OF CELLS. 127 



directly around the nucleus. But cells which from the first have 

 contents (the cell-membrane in these cases forming around masses 

 investing the nucleus), increase very slightly in size. 



The nucleus, and nucleolus also, usually increase in size with the 

 cell; but the nucleoli always retain their globular form, except 

 when dividing fissiparously. 



In cell-growth there is an increase either of the surface or of the 

 thickness of the cell-wall, and this increase may be either general 

 or partial. It is general when the cell grows larger without change 

 of form; 1 partial when the cell changes its form by extending itself 

 at two or more points. Carpenter thinks this extension takes place 

 in the direction of the least resistance ; but its cause is not demon- 

 strated. In some cases the cell becomes narrower as it elongates ; 

 and here we must admit that absorption occurs in one direction 

 while deposition goes on in the other. 



The power of growth does not appear to be simply innate in 

 every organic membrane, and therefore manifesting itself whenever 

 formative material is presented, but it requires certain conditions 

 which the cell-membrane alone affords. The nuclei, when free, 

 never grow to any considerable extent, and especially not in one 

 particular direction. 2 



B. The Nature of the Contents, and the Functions of Cells. 



Usually the contents of a cell may be regarded as a "moderately 

 concentrated solution of albuminous elements with alkaline and 

 earthy salts, and dissolved or suspended fatty particles." Different 

 cells, however, differ greatly in this respect ; some one of these con- 

 stituents greatly predominating in some, while in others altogether 

 different substances may be found. The nerve-cells abound in albu- 

 minous elements ; adipose cells, and those of the sebaceous follicles 

 and of milk-glands, in fat ; while in certain other cells, haBmatine, 

 melanine, or biliary or urinary constituents, abound. 



The functions performed by cells, as inferred from the phenomena 

 manifested by their contents, may be specified under the heads of 



1 According to Schwann, the cell-membrane exerts an attractive influence upon 

 the surrounding fluid, and causes the deposition of newly formed particles in its 

 substance ; and partial growth occurs when the molecules do not all attract equally, 

 but only, or more particularly, at certain points. 



2 The nuclei in the hair-pulp, tooth-pulp, areolar tissue, and smooth muscular 

 fibre, however, grow in one direction, to a considerable extent, these not being free. 



