PHILOSOPHICAL SOCIETY OE WASHINGTON. 63 



So far as we can ascertain, the phenomena of the conduction of 

 nervous impulses by the sensitive nerves are so similar to those of 

 the conduction of motor impulses, that any explanation ultimately 

 adopted for the one will probably apply to the other also. When, 

 however, we ascend to the study of the nervous centres, by which 

 sensitive and motor nerves are connected together, and attempt the 

 interpretation of the complex functions of nerve-cell, ganglion, 

 spinal cord, and brain, we find that none of the hypotheses hitherto 

 brought forward to explain the observed phenomena repose on any 

 defensible chemico-physical basis. 



I cannot, of course, undertake to give to-night even the most 

 meagre outline of the wondrous mechanism which physiological ex- 

 periments show must exist. That reflex actions, co-ordinated muscu- 

 lar movements, and all the complex phenomena of this class, do 

 depend upon a wonderfully complex mechanism, and occur in 

 strict accordance with the ordinary chemical and physical laws, I 

 do not for a moment doubt, and I cordially invite the co-operation 

 of the chemists and physicists to aid the physiologists in the expla- 

 nation of this mechanism, for we stand only upon the threshold as 

 yet. 



If now we turn from the more general discussion of muscular 

 contraction and nervous action, to the consideration of the several 

 functions carried on in animals, by means of special arrangements 

 of the muscular and nervous systems, we continually encounter the 

 preponderating influence of purely physical laws. The introduc- 

 tion of air into the lungs of breathing animals, and its expulsion 

 thence, is effected in a purely mechanical way, while the exchange 

 of the carbon dioxide of the blood with the oxygen of the inspired 

 air occurs in strict obedience to the laws of the diffusion of gases. 



The ordinary laws of hydraulics govern the circulation of the 

 blood and lymph, and all the complex visible motions of the body 

 are executed in accordance with the ordinary laws of mechanics ; 

 nor is it at all necessary for me to insist upon the purely physical 

 nature of the operations of the organs of the special senses, conspic- 

 uously the eye and the ear. For example, so far as concerns the 

 means by which images of external objects are formed sharply upon 

 the retina, the eye is as purely a physical instrument as the telescope 

 or the microscope. But I need not dwell upon this group of phe- 

 nomena, because the importance of the role of the ordinary physical 



