64 THE WORK OF THE DIGESTIVE GLANDS. 



impart the idea of being aware of their purpose, of being conscious 

 of their duty. We have long known that the peripheral endings of 

 sensory nerves are possessed of a high degree of speciality, and 

 cannot therefore have any doubt regarding the specific nature of the 

 end organs of other centripetal nerves. This is a sore point in present- 

 day physiology. But, notwithstanding our knowledge of the separate 

 parts of the animal body, we shall only be able to form a true con- 

 ception of the motive agencies of the whole complicated machine, 

 when we have established the specific excitability of the end apparatus 

 of every centripetal nerve, and have discovered all the mechanical, 

 chemical, and other factors which throw this or that end apparatus 

 into an active condition. I always look upon it as a period of scientific 

 inadequacy so long as the effects of the most diverse external agencies 

 upon any normal physiological process, are admitted to be indistinguish- 

 able. As the work of the digestive canal is now represented in the 

 majority of text-books, and consequently presented to the mind of 

 the physician, it bears the impress of this period. To impart to the 

 physician a more correct conception of this matter was my chief object 

 in giving these lectures. I hope, indeed, to furnish you with evidence 

 sufficiently convincing, that the alimentary canal is endowed not with 

 mere general excitability ; that is to say, does not respond to every con- 

 ceivable form of agency, but only to special conditions which are different 

 for the different portions of its length. Just as men and animals in the 

 world are only able to maintain their existence and constantly adapt 

 themselves to changing circumstances by aid of the peripheral endings of 

 their sensory nerves, so every organ, indeed every cell of every organ, can 

 only maintain its place in the animal microcosm, and adapt itself to the 

 activity of innumerable associates, as well as to the general life of the 

 whole, by virtue of the fact that the peripheral end-apparatus of its 

 centripetal nerves possesses a specific excitability. 



The same applies to the nerve cells : obviously they are endowed 

 with specific sensibility. Irrespective of the excitations which are 

 communicated to them from centripetal nerves, they respond, as 

 originators of nervous impulses, only or at least mainly to definite 

 forms of mechanical, chemical, or other stimuli arising in the organism. 

 This follows not alone from a number of physiological facts but also 

 from various pharmacological data. Thus we learn that various drugs 

 excite or annul the activity of definite portions of the nervous 

 system, at least in the earlier phases of their effects. This specific 

 excitability of nerve cells, just as much as the same property of 

 peripheral end organs, lies at the bottom of the purposive action of 

 these organs. 



Hence, our next duty is to endeavour to discover the normal exciting 



