STRUCTURAL BASIS OF THE HUMAN BODY. 9 



In some cases, however, (myxomycetes) protoplasm can be almost 

 entirely dried up, and is yet capable of renewing its motions when again 

 moistened. 



Dilute salt-solution and many dilute acids and alkalies, stimulate the 

 movements temporarily. 



Ciliary movement is suspended in an .atmosphere of hydrogen or car- 

 bonic acid, and resumed on the admission of air or oxygen. 



5. Electrical. Weak currents stimulate the movement, while strong 

 currents cause the corpuscles to assume a spherical form and to become 

 motionless. 



II. Nutrition. The nutrition of cells will be more appropriately 

 described in the chapters on Secretion and Nutrition. 



Before describing the Reproduction of cells it will be necessary to con- 

 sider their structure more at length. 



Minute Structure of Cells. (a.) Cell-wall. We have seen (p. 5) 

 that the presence of a limiting-membrane is no essential part of the defini- 

 tio7i of a cell. 



In nearly all cells the outer layer of the protoplasm attains a firmer 

 consistency than the deeper portions: the individuality of the cell be- 

 coming more and more clearly marked as this cortical layer becomes more 

 and more differentiated from the deeper portions of cell-substance. Side 

 by side with this physical, there is a gradual chemical differentiation, till 

 at length, as in the case of the fat-cells, we have a definite limiting-mem- 

 brane differing chemically as well as physically from the cell-contents, 

 and remaining as a shriveled-up bladder when they have been removed. 

 Such a membrane is transparent and structureless, flexible, and per- 

 meable to fluids. 



The cell-substance can, therefore, still be nourished by imbibition 

 through the cell- wall. In many cases (especially in fat) a membrane of 

 some toughness is absolutely necessary to give to the tissue the requisite 

 consistency. When these membranes attain a certain degree of thickness 

 and independence they are termed capsules: as examples, we may cite the 

 capsules of cartilage-cells, and the thick, tough envelope of the ovum 

 termed the "primitive chorion." 



(b.) Cell contents. In accordance with their respective ages, positions, 

 and functions, the contents of cells are very varied. 



The original protoplasmic substance may undergo many transforma- 

 tions; thus, in fat-cells we may have oil, or fatty crystals, occupying 

 nearly the whole cell-cavity: in pigment-cells we find granules of pig- 

 ment; in the various gland-cells the elements of their secretions. 

 Moreover, the original protoplasmic contents of the cell may undergo a 

 gradual chemical change with advancing age; thus the protoplasmic cell- 

 substance of the deeper layers of the epidermis becomes gradually con- 

 verted into keratin as the cell approaches the surface. So, too, the orig- 



