56 COUNTER-IRRITANTS 



nerves supplying the subjacent viscera. This theory is sup- 

 ported by the fact that optimum effects are obtained when 

 the skin area stimulated is supplied from the same segment 

 of the spinal cord as that which supplies the viscus to be 

 acted upon reflexly. Thus, when the chest walls are 

 blistered in a case of pleurisy, so soon as the skin becomes 

 hot and tender, a stimulus is conveyed by the afferent nerves 

 to the vaso-motor medullary centres, and thence is reflected 

 by the vaso-motor nerves, causing the pleural capillaries to 

 dilate, thus diminishing tension and pain. 



The several classes of irritants used externally differ 

 materially in the intensity and duration of their effects. 



RUBEFACIENTS produce slight redness and vascular dilata- 

 tion or congestion, and are represented by ammonia solu- 

 tion, iodine, and mild preparations of cantharides ; by 

 alcohol, ether, and chloroform, if evaporation be prevented 

 by oiled silk or other means ; by turpentine and other 

 volatile oils, as well as by smart friction and moderate heat. 

 Owing to the colour of the skin and abundance of hair, 

 reddening in veterinary patients is, however, less obvious 

 than in man. 



MASSAGE or friction with graduated pressure, as in knead- 

 ing or shampooing, exerts many of the effects of counter- 

 irritants. It promotes the circulation of the blood and 

 lymph, hastens absorption of exudates, stimulates the cutan- 

 eous nerves and reflexly the central nervous system, and 

 eases pain. Massage is indicated at the beginning of the 

 treatment of recent strain of ligament or tendon, wrenched 

 joints, oedema, contusion, paralysis, myositis, and muscular 

 atrophy. After removing the hair, the surface of the part is 

 anointed with lard, lanoline, or oil, then rubbed and kneaded 

 for ten or fifteen minutes and always in the direction of 

 venous return. Massage is contra-indicated on septic 

 swellings, abscesses, etc. 



VESICANTS are more active ; cause exudation of plasma, 

 which, collecting under the epidermis, raises vesicles or 

 blisters. Vesicles contain a fluid consisting of about 78 

 parts of water, 18 of albumin, with a little fibrin, and 4 of 

 salts. Steam and boiling water rapidly produce a large 

 amount of effusion. Blisters, by whatsoever agent raised, 



