26 PRINCIPLES OF GENERAL PHYSIOLOGY 



and cannot be identified as of epithelial nature. Similarly, in a fragment of 

 retina, a typical proliferation of the connective tissue fibres does not occur as 

 long as any nervous cells remain alive. 



This mutual effect is not universal, it does not occur in the case of muscle and 

 connective tissue. The facts, as a whole, tend to confirm the point of view expressed 

 above as to the effect of one part of the organism on the growth of other parts. 



Some further details, especially as to the absence of specific influence of the plasma of the 

 same species of animal, will be found on page 288, and in Chapter XXIV. 



The numerous and valuable results obtained by the investigation of chemical changes in 

 surviving tissues and organs, such as muscle and liver, will be dealt with in later pages, when 

 the particular functions in question are under consideration. 



SUMMARY 



Protoplasm in the living state has the properties of a liquid system, containing, 

 however, particles of solids and droplets of immiscible liquids in a freely mov- 

 able state. The protoplasm itself is structureless to the highest powers of the 

 microscope, with ordinary forms of illumination. To the ultra-microscope it 

 presents the characteristics of a colloidal system. 



It forms " organs " for particular purposes ; these organs appear and disappear, 

 according to need. 



But there is no necessity, at present, for the assumption of unknowable " super- 

 mechanical " properties in living cells. Many of the properties referred to can lie 

 explained by known laws, such as those of surface tension, while the time element 

 itself is shown by inorganic colloids. 



By fixing reagents, structure of various kinds, networks, alveoli, and so forth, 

 can be produced. But these structures have no resemblance to the living 

 condition. Obviously, they must be produced from constituents already present, 

 so that certain conclusions are admissible from the examination of fixed cells. 



There is very little ground for the view that protoplasm consists of " biogens " 

 or "giant molecules," in the chemical sense. It is rather a complex of substances 

 of various chemical natures and in various states of aggregation, associated together 

 by forces of surface tension, electrical charge, and so forth. The liquid state enables 

 an elaborate play of forces to take place. Chemical reactions can evidently proceed 

 simultaneously in different parts of a cell, so that there is some mechanism by 

 which one part is isolated from another part, at all events temporarily. After 

 death, this separation ceases to be effective. The activities of the cell are 

 regulated by reversible changes in the distribution of the phases of the complex 

 heterogeneous system of colloids, crystalloids, and solvents. 



For further information on the subjects dealt with in the preceding chapter, 

 the following works may be consulted : 



LITERATURE 



General Properties of Protoplasm. 



Kiihne (1864), pp. 28-108. Von Uexkiill (1909), pp. 11-32. A. Lister (1888). 



Structure of Protoplasm. 



Hardy (1899). Gaidukov (1910). Rhumbler (1914). 



Fixation of Cells. 



Alfred Fischer (1899), pp. 1-72. Hardy (1899). 

 Movements of Protoplasm. 



Ewart(1903). Hormann (1898). Jensen (1902). 



Rhumbler (1898 and 1905). 



Survival and Groivth of Tissues. 



Ross Harrison (1907 and 1910). Carrel and Burrows (1910). 



Methods of Investigation of Unicellular Organisms. 

 Schleip(1911), pp. 1-74. 



The student is advised to read the preceding chapter a second time after having 

 read the following eight or nine chapters. 



