WITH THE MICROSCOPE. 173 



On the other hand, the changes in these pigment-cells of the 

 frog have been considered by Professor Lister to be due to vital 

 actions, and he agrees with Wittich and others who maintain they 

 are under the immediate control of the .nervous system. Indirectly, 

 no doubt, they are, but I do not think that any experiments have 

 proved satisfactorily that the nerves exert any direct influence upon 

 the movements of the particles in these cells. It is well known that 

 the nerves govern the calibre of the vessels, and thus influence the 

 amount of fluid in the surrounding tissues, and in this indirect 

 manner nerves may be said to affect the movements of the particles 

 in the cells. The reader will find a full account of Professor Lister's 

 experiments, and the arguments deduced from them, in his paper 

 " On the Cutaneous Pigmentary System of the Frog," published in 

 the Philosophical Transactions for 1858. 



264. Of Growth and Multiplication. -Almost inseparable from 

 the consideration of the nature of the movements occurring in living 

 things, is the study of operations by which particles are added to 

 and often lifted above other particles in the process of growth, a 

 universal property of things that live. The observer who aims at 

 studying the remarkable and highly interesting phenomena of germi- 

 nation, growth, and multiplication of cells or elementary parts in 

 the tissues and organs of man in health and disease, will find it 

 absolutely necessary to investigate these processes in the simplest 

 living beings where they occur under less complex conditions. He 

 must exercise the utmost caution in drawing inferences from what 

 he does see or rather thinks that he sees, and he must always bear in 

 mind that great and irreconcileable differences of opinion exist among 

 even distinguished observers with regard to the general nature of the 

 changes which takes place when, for example, a spore of common 

 mildew germinates, or an insignificant bacterium gives rise to new 

 bacteria. How then is it likely that the mode of growth, origin, and 

 multiplication of some of the highly complex structures formed in 

 man, especially in the course of disease, can be described with 

 correctness or fully explained to the student ? 



It has been stated over and over again that living bacteria 

 originate in decomposing matters, and one who has recently written 

 on the subject thinks that he has seen the fibrillas of muscle resolve 

 themselves into these living bodies ! It is always necessary to be on 

 our guard against the acceptance of fallacious observations of this 

 kind. Those who have had much experience in the manufacture of 

 pseudo-bacteria, could produce a number of objects and advance 

 facts and arguments which would probably fully convince any 

 inexperienced person that there was abundant evidence to prove 



