lO BIOLOGICAL LECTURES. 



are either covered by a dense and relatively impenetrable mem- 

 brane, or if protected by a barrier less secure, are bathed with 

 fluids at once injurious to them, and offering mechanical 

 obstacles to their settlement. Thus for the exterior of the 

 body we have as a rampart, the skin, and for each orifice its 

 mucous surface with its own peculiar secretion. Even should 

 the parasite be able to pass beyond these outposts, its fate is 

 still insecure, for the lymphatic tissues stand as a closed gate- 

 way to oppose its further progress ; and beyond these again 

 are forces still more impregnable, — the body fluids and cells, 

 which are capable of destroying large numbers of bacteria, 

 and probably no inconsiderable quantities also of their toxic 

 products. 



