158 



BIOLOGY: GENERAL AND MEDICAL 



ing outline of the appearance and evolution of the 

 special senses is offered. 



Touch. The tactile sense can be traced to the irritabil- 

 ity of living substance. It begins without special organs 

 as the phenomenon of thigmotropism. The pseudopods 

 of the rhizopoda are thigmotropic, hence tactile and 

 discriminating. But in composite organization it is not 

 sufficient that the cells shall be equally irritable and 

 similarly impressed by external agents. Division of 



"" Nerve-fibre 



Capsule 



Nerve-fibre 



; Nerve-fibre 



- Nerve-fibre 



FIGS. 58, 59. Meissner's corpuscle from man; X 750. (Bohm, Davidoff, and 

 Huber.) 



labor begins, and it becomes necessary for a more elabo- 

 rate response to follow certain stimuli than could be 

 effected by cells acting individually. Moreover, certain 

 cells are so situated as to be, above their fellows, suscep- 

 tible to external agents, so that we need only ascend to 

 the ccelenterates to find the ectodermal cells more sensi- 

 tive than others, and to find a mechanism by which the 

 external impressions are communicated to groups of 

 cells, by which they are to be utilized, through inter- 

 mediate nerve cells. Though the sensory apparatus is 



