THE TISSUES 



2 9 



t A 



stantaneous ; it takes an appreciable and measurable Nerve cells 

 time. Formerly it was supposed that the nerve fibers, 

 which seem to present no 

 cellular structure, were not 

 parts of cells. We now 

 know that the nerve cell, 

 with its nucleus, is pro- 

 longed into fiber-like exten- 

 sions, reminding us of the 

 pseudopodia of the amiba, 

 but vastly longer, and per- 

 manent. Bundles of these 

 fiber-like filaments consti- 

 tute the nerves. The gen- 

 eral property of irritability 

 is here greatly accentuated, 

 and the impulse is capable 

 of being conveyed to other 

 kinds of cells, which act in 

 consequence of it. So far 

 we seem to be dealing with 

 nothing more than an ex- 

 treme modification of prim- 

 itive functions, but when 

 we come to. regard mental 

 phenomena, especially as 

 found in man, we enter upon 

 a new field. The power of 

 memory may be theoreti- 

 cally explained as analogous 

 to that of the phonograph ; 

 a path of disturbance has 

 left its record in the brain. 

 When we come to conscious- 



FIG. 8. 

 section. 



Drawing by R. Weber 

 Diagrams of gland cells, in 

 The arrows mark the outflow 

 of secretions. A represents the simplest 

 case, in which three cells (shaded) are 

 capable of secreting some substance, 

 which is poured out on to the surface of 

 the body. In B the gland cells secrete 

 into a pocket or tube, which is capable 

 of holding the material until it is 

 wanted, as in the case of the saliva or 

 the secretion of the stomach (gastric 

 juice). This makes it possible to furnish 

 at a given moment much more of the 

 secreted substance than the cells could 

 supply without notice. In plants the 

 gland cells are often situated on a 

 knob or prominence or at the end of a 

 hairlike structure, thus reversing the 

 structure of the tubular gland. 



