12 FIRST LECTURE. 



the Organic, from the infusoria, whose life is counted by- 

 hours, to the oak, whose existence lasts centuries, throughout 

 this limited duration of life. Concerning the human organ- 

 ism, this highest cell-complex, there is a very ancient, well- 

 known saying that it lives seventy years, and at the furthest 

 eighty. 



We now encounter the question : Are the cells, those vital 

 corner-stones of our body, once for all present, to remain 

 with us permanently as faithful companions to the day of 

 death ? Or does our body-cell, that delicate little thing, pos- 

 sess a more limited and, perhaps, compared with human life, 

 only a very short existence ? 



We answer unreservedly in the latter acceptation. 



The life of the body is long, under fortunate circumstances; 

 that of our cells is short. We can present but a very defec- 

 tive proof of this, however, at the present time. 



We again present a few examples. We have said above 

 that the outer surface of our body is covered by layers of cells. 

 The superficial layers are in loose connection ; they are cells 

 in old age. The friction of our clothing daily removes im- 

 mense numbers of them. A cleanly person, who uses sponge 

 and towel energetically every day, rubs off still greater quan- 

 tities. 



This takes place very actively in our mouth every day. 

 We swallow ; our tongue acts in speaking ; drink and food 

 pass this entrance of the digestive apparatus. Every one 

 knows this. The mucous membrane of the mouth is, again, 

 covered with a thick layer of epithelial cells. Here, also, 

 many thousand senile cells are rubbed off daily. That which 

 began at the entrance is continued throughout the entire di- 

 gestive apparatus. An excess of cells is thus lost daily. 



To show the duration of life of a cell variety, let us turn to 

 the human nail. The latter, growing from a fold of skin, is 

 a cell-complex. In the depth of the furrow, youth prevails ; 

 at the upper border — which we trim — old age. The deceased 

 physiologist of Gottingen, Berthold, proved that a nail cell 

 lives four mont'is in summer and five in winter. A person, 



