CHEMICAL SCIENCE. 257 



classifications this peculiar liability of the system might be turned to 

 account as regards the etiology of disease. 



To adduce a characteristic instance of this : when frogs hare been kept 

 for a long space of time in captivity their health declines, and ulcerations 

 arise around the nose and mouth; the nervous system being in this case 

 considerably depressed, the animal is, of course, found to resist much longer 

 the action of strychnia and similar poisons, while parasitical affections spread 

 with fearful rapidity. Frogs are subject to the growth of parasitical fungi, 

 which, after a certain lapse of time, occasion the animal's death. Now, if a 

 healthy frog is placed in a jar containing others affected with the above- 

 mentioned disease, the new-comer sets contagion at defiance; while if 

 another frog, affected with ulcerations in the vicinity of the natural, orifices, 

 is introduced into the jar, the parasitical vegetation covers it at once. 



It has been found that similar affections always have a strong tendency 

 to arise in animals in a low state of health. The itch, a disease which fre- 

 quently prevails among horses and sheep, is scarcely ever found to attack 

 animals in good condition ; and, in man, the lower classes are known to be 

 a prey to vermin, especially in childhood and old age; while persons who 

 live under more favorable circumstances are scarcely ever affected with this 

 inconvenience, except towards the latter end of long and painful diseases; 

 for it is generally in such cases that the morbns pedicular is has been observed. 



The decrease of nervous power equally constitutes a predisposition to 

 putrid, contagious, and virulent affections; the fact is well known to veteri- 

 iiary surgeons. 



It would appear, therefore, that an opposition exists between the two 

 great classes of disease we have just examined; in proportion as the animal 

 grows more sensible to the action of the neurosthenic poisons, the power of 

 resisting the influence of putrid substances is increased. How is this differ- 

 ence to be accounted for? We shall attempt to give you a solution of the 

 difficulty. 



That the chemical composition of the blood should incessantly be modi- 

 fied, is one of the essential conditions of life; repairing, as it does, the daily 

 losses of the economy, and renewing the elements of all the tissues which 

 enter into the system, the blood may be compared to a torrent which con- 

 tinually pours out new substances, while other elements arc flowing into it; 

 and the stronger are the animal's vital powers, the more rapid are the 

 successive changes of the blood; a fact principally observed in birds, which 

 enjoy greater vital energy than any other class of animals. The uninter- 

 rupted continuation of circulation is, therefore, in such animals, of still 

 greater importance than in others; the blood cannot stagnate without 

 promptly acquiring septic properties. If the tributary vessels of a muscle 

 are tied in a mammal or bird, it becomes a putrid mass within twenty -four 

 hours; in a batrachian this change would not take place before a much 

 longer space of time. 



Now, you are aware that the nervous system presides over all the phe- 

 nomena of life in which motion is concerned ; as soon, therefore, as the nerves 

 are impaired, circulation languishes, and the chemical composition of the 

 blood becomes thereby liable to important changes. If, therefore, an animal 

 being given, it is our purpose to preserve it from the action of woorara, 

 or similar poisons, we must lower its forces. If, on the contrary, we intend 



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