Rev. P. Keith on the Susceptibilities of Living Structures. 173 



gredients, — introduce into the veins a portion of atmospheric 

 air, or into the huigs an undue portion of carbonic acid gas, 

 and pain and death ensue. The lacteals in their healthy state 

 admit only chyle ; and the lymphatics, lymph : but in an ex- 

 cited state, that is, in a more sensible state, the former will 

 admit also bile, and the latter blood. Hence the origin of 

 disease : but hence also the possible and occasional means 

 of remedy. Suppose it to be a case of tumour : Give a higher 

 dose, or give the proper dose, of sensibility to the lymphatics, 

 so as to take up and carry off the extravasated matter, and 

 the tumour is resolved. If they will not admit blood in their 

 ordinary state, it is not that their calibre is too small for the 

 globules, as Boerhaave thought ; but because they have not, 

 says Bichat, the proper degree or dose of sensibility*. Add 

 that, and the effect is irresistible. 



Thus there is a degree of sensibility proper to each organ, 

 below and above which its action is faulty, and to which it has 

 a tendency to return. This is its healthy state of action, or its 

 tone or tonicity, by which it resists, to a certain degree, the 

 agency of such causes as are not in unison with that tone, or 

 not materially connected with its functional operation. — Thus 

 also the impressions which are conveyed to the brain through 

 the medium of the external senses, or otherwise, are alto- 

 gether involuntary: they come without our knowledge, and act 

 without our bidding : they are the means by which the in- 

 dividual communicates with the external world, or becomes ac- 

 quainted with the state of the several organs of his own fabric. 

 If the intelligence seems unimportant the individual remains 

 quiescent, and the impression passes off without having pro- 

 duced any notable effect: but if it promises pleasure, or if 

 it portends pain, it instantly generates a volition, and the vo- 

 lition instantly generates some specific action or movement 

 which is rightly said to be voluntary. Thus stimulants fi'om 

 without, and volitions from within, produce upon the several 

 organs a similar effect. Stimulated by the impression of the 

 rays of light, the optic nerve exerts its energies; the impres- 

 sion made is communicated to the brain, and the brain ex- 

 periences the sensation of sight. Stimulated by a volition, 

 the brain, under certain circumstances, exerts its capability of 

 reflecting on its own feelings and ideas. Hence the pha?no- 

 mena of thought and of intellection. It examines, it com- 

 pares, it deduces a conclusion. Under certain other circum- 

 stances, it exerts its capability of generating organic motion ; 

 and hence the plianomena of muscular function as origi- 

 nating in the will. The muscles that are thus excited are 



• Rechcrchcs Pfii/s. p. 66. 



chicflv 



