XXVI INTRODUCTION. 



body is composed, after residing in it for a time, be- 

 come no longer fit for use; their farther residence is, in 

 fact, injurious, and it is necessary to remove them. A 

 system of vessels is provided for this purpose, called 

 the absorbents, which are the scavengers of the body. 

 Taking up, therefore, these eifete atoms, they convey 

 them into the blood vessels, where they are mixed with 

 the common mass of blood. Several organs are pro- 

 vided, as the liver, the kidneys, the surface of the body, 

 and the lungs, through which these effete particles are 

 discharged from the blood in the form of excretions; 

 as the bile, the urine, perspiration, and pulmonary ex- 

 halation. 



We have now sketched the human machine as far as 

 its internal existence, or self-preservation, is concerned 

 in the functions of digestion, circulation, respiration, 

 and excretion. Let us proceed in the inquiry by a 

 rapid glance at those organs by which it is put into a 

 relation with surrounding objects, and on which it de- 

 pends for the sublime operations of the understanding. 



Sensation is derived from the nervous system, com- 

 posed of the brain, the spinal marrow and the nerves. 

 The latter may be traced to many parts of the body, 

 and are supposed to be distributed to all. They main- 

 tain its different sympathies, keep the several organs 

 in one harmonious course of action, and, in some in- 

 stances at least, are indispensable to the performance 

 of their functions. In addition to these, many of the 

 nerves have at their extremities organs of a particular 

 construction, each fashioned in the best manner for the 

 performance of its functions, in making us acquainted 

 with exterior objects. The interior extremities of all 

 these nerves terminate either in the brain or spinal 

 marrow ; the external are the points intended by nature 

 to be affected by the objects around us; but it is indis- 

 pensable to consciousness, that their line of communi- 



