10 THIi VIIAL FUNCIIONS. 



be nourished by them. In the more developed 

 systems of organization this purpose is accom- 

 pHshed by means of tubes, called vessels, through 

 which the nutritive fluids move in a kind of circuit : 

 in this case the function is denominated the Circu- 

 lation. 



It is not enough that the nutritive juices are assi- 

 milated ; another chemical process is still required 

 to perfect their animalization, and to retain them in 

 their proper chemical condition for the purposes of 

 the system. This third object is accomplished by 

 the function of Respiration. 



Fourthly, several chemical products which are 

 wanted in dift'erent parts of the economy, are 

 required to be formed by a peculiar set of organs, 

 of which the intimate structure eludes observation ; 

 although we may perceive that in many instances, 

 among the higher orders of beings, a special appa- 

 ratus of tubes, sometimes spread over the surface of 

 a membrane, at other times collected into distinct 

 masses, is provided for that purpose. These specific 

 organs are termed glands ; and the ofiice performed 

 by them, as well as by the simpler forms of structure 

 above mentioned, is termed Secretion. 



Fifthly, similar processes o( secretion are also 

 cmi)loyed to carry oft' from the blood such animal 

 products as may have been formed or introduced 

 into it, and may possess, or have acquired noxious 

 properties. The elimination of these materials, 

 w hich is the office of the excrctories, constitutes the 

 function of Excretion. 



Sixthly, changes may take place in various parts 

 of the body, both solid and fluid, rendering them 

 unfit to remain in their present situation ; and 



