PHYSIOLOGY. 9 



5. Of the organs employed in receiving and modifying the 

 impressions of external bodies necessary to sensation ; and of 

 their several functions. 



6. Of the motions of the whole body, or of its several parts, 

 which depend on the action of muscles, and not before explained. 

 " Under these six titles we will, I think, exhaust the whole 

 of the functions of animal bodies that are in common to every 

 animal. But the human species, in particular, is distinguished 

 by sexes, which have each of them somewhat peculiar to them- 

 selves. We therefore proceed to treat"" 



7- Of the functions peculiar to the sexes; and of generation. 

 " Physiologists have referred the functions to three classes or 

 heads : The Vital, the Natural, and the Animal Functions. 

 We find some difficulty in referring the different particulars to 

 these heads ; but, in the main, it is an obviously just division. 

 The vital functions are those that are more immediately neces- 

 sary to life, and without the exercise of which, life could not be 

 supported a few moments. With regard to these there have 

 been some differences of opinion. Since the discovery of the 

 circulation of the blood, physiologists have been disposed to 

 consider the action of the heart, and the function of respiration, 

 in this light ; but as the action of the heart itself is dependent 

 on the energy of the brain, this must be considered as a vital 

 function, and the primary one. These, then, form our three 

 first sections. The natural functions are necessary to life ; but 

 they are not so constantly and necessarily required. A man 

 may fast for a length of days without dying, though this con- 

 tinued, to be sure, would destroy him ; and if the aliment is 

 not properly transmitted to the blood-vessels, the same thing 

 will ensue as if the thoracic duct were cut through. They are 

 therefore next in point of necessity to the vital, and form our 

 fourth section. Both the vital and the natural functions are 

 necessary to the being and subsistence of the body. But there , 

 is a certain set of functions for which the body is made and 

 supported, and which form our connexion with the rest of the 

 universe by which we act upon other bodies., and by which 

 other bodies act upon us : these have been called animal func- 

 tions ; and the foundation of the whole of them is to be de- 

 duced from the nature of the nervous system. These animal 



