262 



PROTOPLASM 



fixed material for indications of a fibrous structure in proto- 

 plasm. When the living plasmodium of a slime mold is stretched, 

 the protoplasm (when of sufficiently high viscosity, as it usually 

 is) tears somewhat after the manner of bread dough. Strands 

 within it are conspicuous; as they separate out, they exhibit 

 considerable tensile strength and ultimately snap with sudden- 

 ness (Fig. 139). 



If we turn from protoplasmic qualities to those of tissues, we 

 find again that living matter is built of fibers. The structure 



Fig. 140. — A, fibrous entanglement surrounding a nerve cell {from T. Peterfi 

 after Tschernjachiwsky); B, course of the neurofibrils in a spinal ganglion cell of 

 the frog, as graphically reconstructed by Heidenhain. 



of muscle is fibrous. Nerve tissue is a bundle of threads (Fig. 

 140). The high tensile strength of sinew is an indication of its 

 fibrous character (Fig. 124). 



Certain behavior phenomena are just as significant in indi- 

 cating a fibrous or brush-heap structure of protoplasm as are 

 visible proofs. Pronounced and exceedingly sudden changes 

 in the viscosity of protoplasm may occur when the living sub- 

 stance is disturbed by a microneedle. A striking case of this 

 is that already cited (page 151) in which a cell, in mid-mitosis, 

 with its intricate figure of spindle and astral rays, was pene- 

 trated by a needle, with the result that the entire mitotic figure 

 immediately collapsed, leaving only a relatively homogeneous 

 mass of protoplasm, with no sign of the previously existing cell 

 structures. The collapse is a perfect example of thixotropic 

 behavior. Changes in the viscosity of protoplasm due to 



