INFLAMMATION. 253 



lular organisms which abound in media surrounding us are 

 subject to infectious diseases. Infection, which is one of the 

 most important causes of inflammation, is simply a struggle 

 between a parasite and its host. We must regard the sum 

 total of the phenomena of inflammation in human beings as 

 simply the efforts of nature to offset the effects of an injury. 

 However simple the phenomena resulting from similar injury 

 to a unicellular organism though not constituting inflamma- 

 tion as we are clinically familiar with it yet their significance 

 is the same, and represents a primitive condition of this proc- 

 ess which undergoes a slow evolution as we ascend in the 

 animal scale. The power of locomotion and intracellular 

 digestion is the amoeba's mode of defence. 



Metschnikoff says : " If we take a specimen (such as the 

 yellow plasmodium of Physarum) on our object-glass and 

 touch its central part with a minute glass rod previously 

 heated in a flame, we shall produce thermal excitation. Im- 

 mediately after being touched the central part of the plas- 

 modium dies and may be clearly distinguished from the living 

 peripheral portions, which remain motionless as if nothing 

 has occurred, and are unaffected by the necrosed portion. A 

 few hours later, however, the plasmodium awakes from its 



passive condition and creeps away from the dead part 



We thus see the irritating agencies excite in the plasmodium 

 either a course of events similar to those which accompany the 

 taking of solid nutriment, or a more or less marked repulsion. 

 In attempting to produce a reaction which should correspond 

 to inflammation in the higher animals, we have brought about 

 the phenomena of attraction or repulsion which occur so fre- 

 quently in the lives of plasmodia and the inferior animals 

 generally/ 7 



In embryo sponges there are stages in which the organism is 

 composed of but two layers, the inner of which is formed of 

 amoeboid cells, which have the power of englobing various 

 solid bodies for the purpose of obtaining nutriment and for 

 defence. 



In adult sponges there are three layers ectoderm, mesoderm, 

 and entoderm, and now the function of digestion devolves 

 entirely upon the entoderm, and the amoeboid cells of the 

 mesodern alone possess a phagocytic power. 



