6 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



without going so far, we must allow that they appear 

 to be due to the protoplasm itself; they are self- 

 moved or automatic. 



Living matter, then, is irritable and automatic ; 

 irritability finds expression in contractility, or in the 

 production of such forces as heat, light, or electricity. 



With regard to its general physical and chemi- 

 cal characters, we have to note that it is possessed of 

 great cohesive powers, and yet is very extensile ; it 

 does not mix with water, but it swells by imbibition ; 

 it may expel the contained fluid in the form of 

 rounded vacuoles, and bubbles of gas are sometimes 

 apparent in it. It is ordinarily colourless, and re- 

 fracts light more strongly than water ; it is in most, 

 and probably in all cases, slightly alkaline in reac- 

 tion. 



Before we leave the general consideration of pro- 

 toplasm, we must point out two foreign elements 

 which have to be considered. The first of these is the 

 presence in protoplasm, as we ordinarily observe it, of 

 various more simple chemical compounds, which have 

 the form of granules ; these, which may be fatty or 

 starchy bodies, are conveniently grouped together 

 under the head of metaplasm; they may be re- 

 garded as owing their origin to the changes that are 

 constantly taking place in the molecular constitution of 

 the protoplasm, or, in other words, as waste products 

 not yet eliminated. 



The second is a general motion of a protoplasmic 

 mass, especially when of particularly small size (e.g. 

 bacteria) ; this movement of the body as a whole is 

 not a vital, but a purely physical phenomenon, as may 

 be demonstrated by the simple experiment of rubbing 

 up a little gamboge in a drop of water, when exactly 

 the same movement is to be observed. This approxima- 

 tion and separation of small particles is a phenomenon 

 which has attracted the attention of the physicist, by 



