FUNCTIONS OF MAN. 71 



is concerned, as light in vision ; sound in audition ; odours in olfaction ; 

 tastes in gustation. The properties of these agents will, in all instances, 

 be detailed in a brief manner. 



The difficulty of observing actions, that are carried on by the very 

 molecules of which the organs are composed, has given rise to many 

 hypothetical speculations, some of which are sufficiently ingenious; 

 others too fanciful to be indulged for a moment; and, as might be 

 expected, the number of these fantasies generally bears a direct pro- 

 portion to the difficulty and obscurity of the subject. It will not be 

 proper to pass over the most prominent of these, but they will not be 

 dwelt upon ; whilst the results of direct observation and experiment 

 will be fully detailed; and where differences exist amongst observers, 

 such differences will be reconciled, where practicable. 



The functions, executed by different organs of the body, can be de- 

 duced by direct observation; although the minute and molecular action, 

 by which they are accomplished in the very tissue of the organ, may 

 not admit of detection. We see blood proceeding to the liver, and the 

 vessels that convey it ramifying in the texture of that viscus, and 

 becoming so minute as to escape detection even when the eye is aided 

 by a powerful microscope. We find, again, other canals in the organ 

 becoming perceptible, gradually augmenting in size, and ultimately 

 terminating in a larger duct, which opens into the small intestine. If 

 we examine each of these orders of vessels in their most minute appre- 

 ciable ramifications, we discover, in the one, always blood ; and, in the 

 other, always a very different fluid, bile. We are hence led to the 

 conclusion, that in the intimate tissue of the liver, and in some part 

 communicating directly or indirectly with both these orders of vessels, 

 bile is separated from the blood ; or that the liver is the organ of the 

 biliary secretion. On the other hand, functions exist, which cannot 

 be so demonstratively referred to a special organ. We have every 

 reason for believing, that the brain is the exclusive organ of the mental 

 and moral manifestations ; but, as few opportunities occur for seeing it 

 in action; and as the operation is too molecular to admit of direct 

 observation when we do see it, we are compelled to connect the organ 

 and function by a process of reasoning only ; yet, we shall find, that 

 the results at which we arrive in this manner are often by no means 

 the least satisfactory. 



The forces which preside over the various functions are either gene- 

 ral, that is, physical or chemical ; or special, that is, organic or vital. 

 Some of the organs afford us examples of purely physical instruments. 

 We have in the eye, an eye-glass of admirable construction; in the 

 organ of voice, an instrument of music ; in the ear, one of acoustics : 

 the circulation is carried on through an ingenious hydraulic apparatus ; 

 and station and progression involve various laws of mechanics. In 

 many of the functions, again, we have examples of chemical agency, 

 whilst all in which innervation is concerned are incapable of being 

 explained on any physical or chemical principle; and we are constrained 

 to esteem them vital. 



