342 EXCRETION". 



under which those materials present themselves, which have once 

 formed a part of the living tissues, but which have become altered 

 by the incessant changes characteristic of organized bodies, and 

 which are consequently no longer capable of exhibiting vital pro- 

 perties, or of performing the vital functions. They are, therefore, 

 destined to be removed and discharged from the animal frame, and 

 are known accordingly by the name of Excrementitious Substances. 



These excrementitious substances have peculiar characters by 

 which they may be distinguished from the other ingredients of the 

 living body ; and they might, therefore, be made to constitute a 

 fourth class of proximate principles, in addition to the three which 

 we have enumerated in the preceding chapters. They are all sub- 

 stances of definite chemical composition, and all susceptible of 

 crystallization. Some of the most important of them contain nitro- 

 gen, while a few are non-nitrogenous in their composition. They 

 originate in the interior of living bodies, and are not found else- 

 where, except occasionally as the result of decomposition. They 

 are nearly all soluble in water, and are soluble without exception in 

 the animal fluids. They are formed in the substance of the tissues, 

 from which they are absorbed by the blood, to be afterward conveyed 

 by the circulating fluid to certain excretory organs, particularly the 

 kidneys, from which they are finally discharged and expelled from 

 the body. This entire process, made up of the production of the 

 excrementitious substances, their absorption by the blood, and their 

 final elimination, is known as the process of excretion. 



The importance of this process to the maintenance of life is readily 

 shown by the injurious effects which follow upon its disturbance. 

 If the discharge of the excrementitious substances be in any way 

 impeded or suspended, these substances accumulate, either in the 

 blood or in the tissues, or in both. In consequence of this retention 

 and accumulation, they become poisonous, and rapidly produce a 

 derangement of the vital functions. Their influence is principally 

 exerted upon the nervous system, through which they produce 

 most frequent irritability, disturbance of the special senses, deli- 

 rium, insensibility, coma, and finally death. The readiness with 

 which these effects are produced depends on the character of the 

 excrementitious substance, and the rapidity with which it is pro- 

 duced in the body. Thus, if the elimination of carbonic acid be 

 stopped, by overloading the atmosphere with an abundance of the 

 same gas, death takes place at the end of a few minutes ; but if the 

 elimination of urea by the kidneys be checked, it requires three or 



