62 



THE CELL 



rheumatic individuals with different conditions of weather. We know very 

 little at this time about the real nature of these agents. Recently Arrhenius 

 has sought to bring various physiological processes, notably menstruation, into 

 relation with electrical variations of the atmosphere and the chemical changes 

 thereby effected. But the results thus far obtained on this subject appear to 

 be too limited to justify a fuller presentation in this book. 



I. CONDUCTIVITY 



Besides these artificial stimuli which are able to excite the cells or to 

 increase their excitability, there occurs in the Metazoa a form of stimulus 

 which belongs to the body itself, namely, the stimulation of one cell by an- 

 other. It is in this connection that the nerves 

 are of the very greatest importance ; they trans- 

 fer their excitations to the end organs, the mus- 

 cle cells, gland cells, etc. ; or they are themselves 

 stimulated by other cells, as when the sensory 

 nerve fibers are roused to activity by their 

 peripheral end organs, or when a nerve cell by 

 means of its processes arouses another cell to a 

 state of activity. Here belongs also the case 

 of the stimulation of a smooth muscle cell by 

 its neighbor, which in all probability is accom- 

 plished by means of the protoplasmic connec- 

 tions (intercellular bridges) which have been 

 demonstrated between these as well as between 



FIG 36.-Cross section of the in- other kindg of ^ (cf. Fig. 36). This form 

 testmal musculature of the cat, ' 



after Boheman. The dotted of stimulation represents one of the most mi- 

 areas represent cross sections portant mechanisms by which the different 

 of muscle-cells, the black lines partg of the Metazoan body are made to coop- 

 between them represent inter- , . , 

 cellular bridges. erate harmoniously. 



The stimulus is transmitted from one part 



to the other also in a single cell. The clearest example of this we have again 

 in the nerves, which are nothing else than long processes of nerve cells, and 

 they propagate a stimulus directly transmitted to them by passing it along 

 from one section to the next throughout its entire length. We meet with the 

 same mode of transmission wherever a cell is stimulated at one definite point 

 and the excitation extends throughout the entire cell body. 



J. THE ASSIMILATIVE PROCESSES INDUCED BY STIMULATION 



Our knowledge of the assimilative processes induced by different stimuli 

 is still very imperfect. 



With respect to the question of most interest to us, namely, the influence 

 of stimuli upon the formation of living substance, we know that the Bacteria 

 and Infusoria multiply rapidly as the result of increasing the supply of nour- 

 ishment, and that the sunlight gives the impetus for the formation of the, 



