PROTOPLASMIC FUNCTION 



41 



structure which is essentially a smooth muscle cell. Within this minute 

 organ very fine contractile fibers are to be found, known as myoids, and 

 it is this elementary thread-like mass of protoplasm, capable of short- 

 ening, which seems to be characteristic of all the varied sorts of muscle 

 described in a separate chapter below. 



Besides movements due to active contraction and the passive relax- 

 ation of protoplasm, there are others of much less frequent occurrence, 



FIG. 14 



FIG. 15 



Epithelial cells from the human trachea, 

 ciliated and goblet cells: a, 3 a, 2 and a 1 , various 

 stages in the development of the ciliated cells; 

 b and ft 1 , goblet cells; d, the upper ends of 

 two developed goblet cells. (Behrens, Kossel, 

 and Schiefferdecker.) 



and therefore of less importance. 

 One of these is caused by changes 

 in the specific gravity, the animal 

 cell being made lighter by the 

 secretion of a bubble of carbon 

 dioxide, and then at another 

 time heavier than water by the 

 expulsion of this bubble of gas. 

 Again, certain forms progress by 

 the continuous secretion of a 

 viscid substance from the end of 



the body Which is in Contact With Some different types of epithelial cells: a, 



the Surface along which it moves. ciliated epithelium; 6, cylindrical, seen in profile; 



/->, A . n d, the same seen on the end; f , cells with flagellae 



SECRETION. Another mamfes- and " co ii are ttes; g . flagellate ceils; h, digestive 



tation Of the irritability Of protO- cells with ameboid projections (as in hydra); i, 



plasm is to. be seen in the general stratified e P ithelium '.. * external epithelium of a 



1 . _. marine planarian with pigment -cells, epithelial 



function 01 Secretion. In SOme cells, and gland-cells beneath. (Dubois.) 



form or other this is a universal 



activity in protoplasm. Secretion may be said to be the process by 



which protoplasm produces within itself by its own activity some sub- 



