30 CELLS AND EXTRACELLULAR ELEMENTS. 



Thus the functions of such cells become distributed and dif- 

 ferentiated: one set of cells, in the muscular system, exercises 

 the power of contractility ; others, in the nervous system, are 

 specially arranged as seats of sensibility and consciousness ; 

 other cells have chemical functions, in the secretory appara- 

 tus ; some build up the framework of the body, some serve as 

 a covering to the surface, others provide for reproduction. 



Movements of cells : The chief visible vital movements, of 

 automatic or quasi-spontaneous character, of which cells are 

 capable are: 1, amoeboid movement; 2, contraction; 3, cili- 

 ary movement ; 4, cyclosis, or internal circulation. These are 

 all probably varying manifestations of one fundamental mode 

 of motion. Besides these, a purely physical, not vital, micro- 

 scopic motion is sometimes observed in cells, namely, 5, 

 Brown ian movement. 



Amoeboid movement : Many free cells, as the leukocytes in 

 the blood and some of the protozoa, possess the power of 

 spontaneously protruding and withdrawing processes of the 

 body-protoplasm. These processes are called pseudopod ia ; 

 and this kind of movement is called amoeboid, from the fact 

 that it is characteristic of the protozoon known as the amoeba. 

 At rest, the cells capable of amoeboid movement are usually 

 spheroidal ; but when in motion their shape may become very 

 irregular. 



The pseudopodia have no definite forms or size, but are very 

 variable and irregular. They may be broad, or they may be 

 slender and filamentary ; they are always temporary and capa- 

 ble of retraction, unlike permanent cell-processes. By means 

 of them the cells are enabled to seize food-particles, or to 

 move themselves about, extending a pseudopodium in one 

 direction, and then by enlarging it the body gradually follows. 



The hyaloplasm, or clear outer part of the protoplasm, is 

 the more actively concerned in the amoeboid movements ; it is 

 the first to be protruded in the pseudopodia, while the granu- 

 lar portion of the protoplasm more slowly follows. The 

 throwing out of the pseudopodia is said to be due to currents 

 of the fluid protoplasm, which, impinging on the periphery 

 of the cell, unbounded by a stiff wall, cause it to bulge and 

 protrude at that point. 



Contraction is a mode of movement, closely allied to amce- 



