340 INDUCTION. 



probably never reaches those nerves. Again, " it has been observed that 

 vegetable gum is not digested in the stomach; the coats of that organ 

 dialyse the soluble food, absorbing crystalloids, and rejecting all colloids." 

 One of the mysterious processes accompanying digestion, the secretion of 

 free muriatic acid by the coats of the stomach, obtains a probable hypo- 

 thetical explanation through the same law. Finally, much light is thrown 

 upon the observed phenomena of osmose (the passage of fluids outward 

 and inward through animal membranes) by the fact that the membranes 

 are colloidal. In consequence, the water and saline solutions contained in 

 the animal body pass easily and rapidly through the membranes, while the 

 substances directly applicable to nutrition, which are mostly colloidal, are 

 detained by them.* 



The property which salt possesses of preserving animal substances from 

 putrefaction is resolved by Liebig into two more general laws, the strong 

 attraction of salt for water, and the necessity of the presence of water as a 

 condition of putrefaction. The intermediate phenomenon which is inter- 

 polated between the remote cause and the effect, can here be not merely 

 inferred but seen ; for it is a familiar fact, that flesh upon which salt has 

 been thrown is speedily found swimming in brine. 



The second of the two factors (as they may be termed) into which the 

 preceding law has been resolved, the necessity of water to putrefaction, 

 itself affords an additional example of the Resolution of Laws. The law 

 itself is proved by the Method of Difference, since flesh completely dried 

 and kept in a dry atmosphere does not putrefy ; as we see in the case of 

 dried provisions and human bodies in very dry climates. A deductive 

 explanation of this same law results from Liebig's speculations. The 

 putrefaction of animal and other azotized bodies is a chemical process, by 

 which they are gradually dissipated in a gaseous form, chiefly in that of 

 carbonic acid and ammonia ; now to convert the carbon of the animal sub- 

 stance into carbonic acid requires oxygen, and to convert the azote into 

 ammonia requires hydrogen, which are the elements of water. The ex- 

 treme rapidity of the puti-efaction of azotized substances, compared with 

 the gradual decay of non-azotized bodies (such as wood and the like) by 

 the action of oxygen alone, he explains from the general law that sub- 

 stances ai'e much more easily decomposed by the action of two different 

 afiinities upon two of their elements than by the action of only one. 



§ 3. Among the many important properties of the nervous system which 

 have either been first discovered or strikingly illustrated by Dr. Brown- 

 Sequard, I select the reflex influence of the nervous system on nutrition 

 and secretion. By reflex nervous action is meant, action which one part 

 of the nervous system exerts over another part, without any intermediate 

 action on the brain, and consequently without consciousness ; or which, if 

 it does pass through the brain, at least produces its effects independently 

 of the will. There are many experiments which prove that irritation of a 

 nerve in one part of the body may in this manner excite powerful action 

 in another part; for example, food injected into the stomach through a 

 divided oesophagus, nevertheless produces secretion of saliva ; warm water 

 injected into the bowels, and various other irritations of the lower intes- 

 tines, have been found to excite secretion of the gastric juice, and so forth. 



* Vide Memoir by Thomas Graham, F.R.S., Master of the Mint, "On Liquid Diffusion 

 applied to Analysis," in the Philosophical Transactions for 1862, reprinted in the Journal of 

 the Chemical Society/, and also separately as a pamphlet. 



