ANATOMY AND PHYSIOLOGY 15 



Anatomy and Physiology. 



Certain differences in the action of medicines may be observed as 

 occurring in the various species of animals, and in animals as contrasted 

 in this respect with man. 



Action of Drugs on Animals as Compared with That on Man. 

 !From a comparative standpoint the action of drugs on the nervous 

 system of animals differs from that on man. This follows according to 

 the "law of dissolution," which teaches that the more highly developed a 

 part of the nervous system is in the evolutionary scale, the more sensi- 

 tive is it to the influence of drugs. Since the cerebrum of man is rela- 

 tively larger and more highly developed, in proportion to his weight, 

 than is the case in animals, and since the spinal cord is larger and more 

 highly developed in proportion to the brain in animals, it happens that 

 drugs impressing the nervous system exert less effect on the brain, and 

 more on the spinal cord, of animals than in man. 



Thus opium is more powerful in its influence on the brain of man, 

 and strychnine is more potent in its action on the spinal cord of animals. 

 Drugs are not absorbed so rapidly or perfectly in the enormous digestive 

 apparatus of ruminants as in man ; neither do emetics act in these animals, 

 nor in horses ; while in none of the lower animals are agents causing 

 sweating so efiScient as in man. 



Action of Drugs on Horses as Compared with That on Other 



Animals. 

 Differences exist relative to the action of drugs on the horse, as com- 

 pared with other animals, chiefly in respect to the digestive apparatus. 

 Emetics do not act on the horse, as this animal does not vomit unless the 

 stomach is greatly distended with gas which causes dilatation of the 

 cardiac outlet. Moreover, the stomach is too small to be successfully 

 compressed by the abdominal walls, and the great length of the esophagus 

 between the stomach and diaphragm, together with the horseshoe-like 

 band of fibres at its cardiac extremity, prevent the regurgitation of food. 

 The intestines of the horse, on the other hand, are as voluminous as the 

 stomach is greatly distended with gas which causes dilatation of the 

 (as purgatives), although the action of cathartics is slow. The bowels 

 of horses excrete vastly more of the fluid ingested than is the case in 

 man or dogs — whose kidneys chiefly assume this function — and the kid- 

 neys are said to eliminate about 15 per cent, of the fluid ingesta in horses, 

 as against 50 per cent, in man and dogs. 



Action of Drugs on Ruminants as Compared with That on Other 



Animals. 



The capacious four-fold stomach of ruminants always contains large 

 amounts of food in the rumen and abomasum, while the impervious, 

 poorly vascular and skin-like gastric mucous membrane renders absorp- 

 tion feeble and imperfect and enforces a comparatively larger dosage 

 than is proper for horses of greater weight. Ruminants are also gener- 

 ally insusceptible to emetics. The skin and kidneys of ruminants are' 

 still less active than is the case in horses. 



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