392 PROCESSES OF LIFE REVEALED BY THE MICROSCOPE. 



defined cells and nuclei, the other with the cells clouded, filled with 

 grannies and with the outlines of cells and their nuclei almost indis- 

 cernible. Between tbese there might be various gradations in the dif- 

 ferent forms. And yet, from what has been stated above, it is plain 

 that all these different structural appearances represent phases of 

 activity, and all might have come from the self-same animal. In like 

 manner, if certain parts of the nervous system were to be studied com- 

 paratively, and the tissue taken from one animal after refreshing sleep 

 and rest, from another after exhausting labor, another in infancy, and 

 another from an animal decrepit with years, the difference in general 

 appearance and in structural details would be striking enough to 

 satisfy any morphologist that, as with the structure of the pancreatic 

 cells, there were two or more distinct types; but the physiological his- 

 tologist would recognize at once that the differences so much insisted 

 upon represented different phases of activity, and, as with the pan- 

 creatic cells, might be all represented in the same animal at different 

 times. 



I would be far from saying that there are no structural differences 

 in the different animals independent of any particular phase of func- 

 tional activity; but if these only are sought and the others neglected 

 the physiological appearances will often obtrude and confuse, if they 

 do not utterly confound. 



I have, therefore, for the last ten years urged my students, and mean 

 to go on advocating with all the earnestness of which I am capable, 

 that in studying an organism or its tissues, the investigator, to gain 

 certain knowledge, must know all that it is possible to learn concerning 

 the age, health, state of nervous, muscular, and digestive activity; in 

 fact, all that it is possible to find out about the processes of life that 

 are going on and have gone on when the study is made. 



There are some microscopic forms in which the entire study can be 

 made while the creature is alive. With the higher organisms, also, 

 some of the living elements, as the white-blood corpuscles and ciliated 

 cells, can be studied, and their various actions and structural changes 

 observed for a considerable time. 



The white-blood corpuscles or leucocytes resemble the amoeba very 

 closely in their actions and powers, as we have seen in discussing the 

 way in which the body is freed from foreign particles. The ciliated 

 cells are among the most striking of all the constituent elements of the 

 body. One end is fixed firmly to the tissues, the sides are in contact 

 with their fellow cells, but the other end is free and bears great num- 

 bers of hair-like processes, the cilia, which project freely into somecav- 

 ity or upon some surface. What histologist would be able for a moment 

 to suggest the p3wer of these hair like processes if he studied the dead 

 cells alone? Yet the moment these cells arc studied alive under the 

 microscope it is seen that for the service of the body all the powers of 

 these cells are concentrated into one, that of motion, and all the motion 



