SKIN AND MUCOUS MEMBRANES 147 



the greater penetrability of the vessel walls, but the chemotactic 

 action of the chemical irritants, which attracts the leucocytes, sets 

 up the amoeboid movement and causes them to wander out to the 

 chemotactic centre (cantharides, croton oil). The emigrated leu- 

 cocytes act as phagocytes (Metschnikoff) and also produce a diges- 

 tive enzjTne which transforms the disease products into albumoses 

 (propeptones) and by this action liquefies them (histolysis). The 

 term digestives, formerly applied in surgery to those drugs which 

 stimulated suppuration, was therefore entirely warranted. The 

 bacteria-free pus produced by the digestives possesses a strong bac- 

 tericidal action. There is also probably an increased local forma- 

 tion of antitoxins in consequence of stimulation of the tissue cells 

 by the aeries. (See the chapter on hydrotherapy.) According to 

 the force of the chemical irritation, different degrees of cutaneous 

 inflammation are produced: hypersemia, vesicle and pus formation. 

 Another, although smaller, division of cutaneous irritants acts like 

 the caustics by coagulation or solution of the albumin of the tissues. 

 (See the following chapter on caustics.) 



The skin and mucous membranes differ very greatly in their 

 sensitiveness to the aeries. The reason for this is to be found in 

 the difference in the covering epithelium. The horny, dry, dead 

 cells of the epiderm protect the skin against the action of dry, 

 solid irritants, so that it is indifferent, for example, to sublimate, 

 silver nitrate, arsenic and even to phosphorus in the dry condi- 

 tion, but when these substances are applied to the tender, moist 

 cells of the mucous membrane, with its covering of fluid, they are 

 immediately dissolved and penetrate the membrane. On the 

 other hand, the skin is more susceptible than the mucous mem- 

 branes to some of the fluid irritants, especially those which are 

 volatile. Turpentine oil, for instance, is less irritant to the tracheal 

 and intestinal mucous membranes than to the external skin because 

 the mucous membranes are covered with a protective layer of 

 mucus which the ethereal oil, not being miscible with water, 

 cannot penetrate, while it is quickly imbibed by the dry cells 

 of the epiderm and can also rapidly pass through the same in the 

 gaseous form. 



