INTRODUCTION. 45 



particles, and the results of the cutaneous ab- 

 sorption. 



That the blood may be in a fit state to nourish 

 the respective parts of the body, it is necessary 

 that by the action of respiration, through the me- 

 dium of the surrounding element, it should undergo 

 the modification before alluded to. In animals pos- 

 sessing a circulating system, a portion of the ves- 

 sels are destined to carry the blood into certain or- 

 gans, where it is thinly spread over a considerable 

 surface, the better to receive the necessary action 

 of the surrounding element. When the local habi- 

 tation of the animal is the land, this surface is hol- 

 low, and is called the lungs ; but it juts out, when 

 the animal resides in the water, and is then termed 

 gills. There are always certain organs of motion 

 provided to propel the surrounding element, whe- 

 ther air or water, into or upon the organs of respi- 

 ration. 



In animals without a circulating system the air 

 is diffused through all the parts of the body by 

 elastic vessels called trachea?, or the water acts 

 upon them either by penetrating the body by means 

 of certain vessels, or simply by bathing the surface 

 of the skin. 



The blood, after having been subjected to the 

 action of respiration upon it, is in a proper state to 

 renovate the composition of the various parts of the 

 body, and to perform the function of nutrition pro- 

 perly so called. The facility the blood possesses 



