4 PHYSIOLOGY OF MUSCLES AND NERVES. 



attention. If portions of vegetable or animal bodies 

 are observed under high magnifying powers, small 

 granules or similar bodies are seen to be engaged in a 

 peculiar tremulous motion. Whence does this arise ? 

 That it is not a vital phenomenon is sufficiently shown 

 by the fact that perfectly inanimate bodies, for instance, 

 the carbon particles of finely rubbed Indian ink, exhibit 

 the same movement. The effect is, in fact, due merely 

 to currents in the fluid, by which the light particles 

 suspended in the fluid are carried away. Such currents 

 are easily engendered in any fluid, sometimes in con- 

 sequence of uneven temperature, sometimes in conse- 

 quence of evaporation, sometimes, also, as the result of 

 the unavoidable shaking of the microscope. Weak as 

 these currents may be, the disturbance caused by them, 

 when seen under strong microscopic power, seems con- 

 siderable, and is often hardly distinguishable from those 

 movements which are caused by the vital activities of 

 the particles. Sometimes this molecular motion may 

 be detected within parts of living bodies ; in which case 

 small granules swim about in a clear fluid within larger 

 or smaller cavities in these parts of living bodies. 



4. If a drop of pond water is placed under the 

 microscope, many living objects, some of which shoot 

 quickly about in all directions, are usually discernible 

 in the water. Side by side with these occur certain 

 oblong, or rod-shaped bodies, moving tremulously about 

 with greater or less rapidity. It is often hard to 

 distinguish whether the motion seen in these latter is 

 independent or molecular. It must be observed whether 

 of these bodies two contiguous individuals always pass 

 along in the same direction, or whether their move- 

 ments appear independent of each other. In the latter 



