PROTOPLASM AS A PHYSICAL SYSTEM 6i 



breakdown is increased, and the restoration of lilm- 

 structure between successive contractions becomes im- 

 perfect, with the result that eventually the whole struc- 

 ture disintegrates. These effects may be compared with 

 those of excessive fatigue, which also leads to irreparable 

 structural breakdown.^ 



STRUCTURE OF PROTOPLASM 



It will be evident from the preceding discussion that 

 structure is only one factor in the chemical activity of 

 protoplasm; undoubtedly many other factors— those 

 entering in all chemical reactions, such as concentration, 

 temperature, special affinities, catalysis — enter in deter- 

 mining the rate and character of the metabolic reactions. 

 But the controlling factor— that which is subject to 

 rapid and reversible alteration under the intluence of 

 stimulating agencies — appears to be the peculiar structure 

 of the living substance. By the conception of ''struc- 

 ture" as applied to protoplasm is meant, generally 

 speaking, the distribution of the physically stabler 

 components, usually the solid components, of the system. 

 Evidently, as already pointed out, this structure is 

 itself a product of metabolism; but having once been 

 formed, it influences the further course of metabolism— in 

 the general manner of which Child's comparison of the 

 living organism to the flowing river'' gives a good illustra- 

 tion by analogy. In the living organism there is always 

 structure of a definite kind; even the simplest "undilTcr- 



^ Cf. the instances of structural alterations in the central nervous 

 system described in Crile's recent book, ,1 Physical Interpretation of 

 Shock y Exhaustion, and Restoration, London (1921). 



^ Cf. Child, The Regulatory Process in Organisms, Journal of Mor- 

 phology, XXII (1911), 171; also Senescence and Rejuvenescence, chap. i. 



