25 



lightning, provided the relation of the latter with the series 

 ceased with its passing into the bone. 



39. But although this distinction seems a very obvious one, 

 yet there is often much difficulty, nay, it is sometimes impossible 

 to make it: we may define the principle of the distinction, and 

 have no doubts about it; but we cannot assert the truth of our 

 limitation, if it should be required, in all particular instances. 

 Thus, for example, a drop. of oil of vitriol, applied to the sciatic 

 nerve pretty near its origin, would produce a convulsion of all the 

 muscles, and perhaps destroy life. Now the properties of the 

 oil of vitriol are related with those of the nerve, these latter with 

 those of the spinal marrow, these last with those of the whole 

 muscular system. Where then, I would ask, does the agency of 

 the properties of the oil of vitriol cease? They produce a change 

 in those of the nerve, which change is communicated to the pro- 

 perties of the spinal marrow, producing a change in them; what 

 then is the relation between the oil of vitriol combined with these 

 changed properties of the nerve, and the properties of the spinal 

 juarrow? The mode itself of causation is extremely simple, yet, 

 from the interchange of the process in phenomena, it becomes 

 inscrutably complex. Let us however pursue the question a little 

 further. 



40. The properties of the oil of vitriol are so related with 

 those of the nerve, that the latter are made, by combination with 

 the former, an altered identity: this identity has relation with the 

 properties of the spinal marrow. Now this relation must agree with 

 the modes of causation: it may be, that the spinal marrow is in- 

 fluenced by more properties than usual, or by fewer properties 

 than usual: it may be influenced by addition or by privation. 

 Thus, in the natural state, properties of the nerve may be per- 

 petually passing from the nerve to the spinal marrow, as they are 

 from the spinal marrow to the nerve; the combination which is 

 produced by the oil of vitriol and the properties of the nerve, may 

 have no relation with the spinal marrow; the office of this last 

 would then be changed for want of the usual influence : or the 

 relation of the oil of vitriol may be only with some of the proper- 

 ties of the nerve, admitting the usual communication of others to 

 the spinal marrow, this partial supply of usual properties would 

 derange the function of the spinal marrow by privation ; or, the 

 relation of the acid may be with all the properties of the nerve, 

 and the combination thus formed, may be related with the spinal 

 marrow ; then the relation of the properties of the oil of vitriol, 

 thus communicated to the spinal marrow, may be with all or with 

 some of its properties, thus modifying the result; and the relation 

 of the acid may either stop with all, or some of the properties of 

 the spinal marrow: or the properties of the former, conjointly with 

 those of the latter, may be extended to the muscular system. 

 Thus we see what difficulties oppose the attainment of a precise 

 philosophy, which emulates the knowledge of the causes which 



