6 INTRODUCTION. 



ment, and nerves for sensation. Respiration is another essential 

 function in the animal constitution ; and in proportion as the re- 

 spiratory system is complete, the animal functions are more fully 

 exercised. In addition also to the chemical elements which en- 

 ter into the composition of vegetables oxygen, hydrogen, and 

 carbon a fourth substance, azote, seems almost peculiar to the 

 animal constitution. To complete the distinction between ani- 

 mal and vegetable life, Hedwig has ingeniously remarked, that 

 in vegetables the sexual organs fall each year, or at each pro- 

 duction, while animals preserve them through the whole course 

 of their existence, 



As nutrition is the most general function of living bodies, 

 under the name of organs of nutrition are comprehended all 

 the parts of the body by which alimentary matters are intro- 

 duced for its support, or which are employed in preparing the 

 food for that purpose. The materials of nutrition penetrate by 

 various means into organized bodies. They may either be in- 

 troduced under the form of elastic fluids by the pores or imper- 

 ceptible interstices in all living bodies, or they may be conveyed 

 by a particular organization for this purpose into an internal or- 

 gan of digestion. Sometimes this internal canal or digestive ca- 

 vity has the form of a tube with two orifices, the one for the en- 

 trance of food, the,other for the exit of matters unfit for the pur- 

 poses of life ; others have only a single opening destined to this 

 double use ; and a few which are found in water absorb their 

 nourishment in the manner of vegetables, with this difference, 

 that the canals which run from these numerous mouths end in 

 a common cavity. The solid matters introduced into the di- 

 gestive cavity or stomach are converted by an internal process 

 first into a pulpy mass named chyme, and afterwards into a semi- 

 fluid substance denominated chyle, which is finally taken up 

 or absorbed by appropriate vessels, and conveyed to the great 

 centre of circulation, the heart. 



The movement communicated by the action of the heart to 

 the internal fluids, now mixed with other animal liquids and 

 termed blood, by which they are impelled through the body, 

 is known by the name of circulation. The vessels which 

 conduct the blood or chyle to the heart are called veins ; those 

 which conduct it from the heart to the other parts of the body 



