INTRODUCTION. 23 



It would appear then that these causes act on the nervous fluid 

 chemically, and by changing its composition ; this appears the more 

 likely, as their action becomes weakened by continuance, as if the 

 nervous fluid needed the resumption of its primitive composition, to 

 fit it for a fresh alteration. 



The external organs of the senses may be compared to sieves, 

 which allow nothing to pass through to the nerve, except that species 

 of agent which should affect it in that particular place, but which 

 often accumulates it so as to increase its effect. The tongue has its 

 spongy papillae which imbibe saline solutions ; the ear, a gelatinous 

 pulp which is violently agitated by sonorous vibrations ; the eye, 

 transparent lenses which concentrate the rays of light, &c. &c. 



It is probable, that what are styled irritants, or the agents which 

 occasion the contractions of the fibre, exert this action by producing 

 on the fibre, by the nerve, a similar effect to that produced on it by 

 the will ; that is, by altering the nervous fluid, in the way that is re- 

 quisite to change the dimensions of the fibre which it influences : 

 but with this process the will has nothing to do, and very often the 

 ME is entirely ignorant of it. The muscles separated from the body 

 preserve their susceptibility of irritation, as long as the portion of 

 the nerve that remains with them preserves the power of acting on 

 them — with this phenomenon the will has evidently no connexion. 



The nervous fluid is altered by muscular irritation, as well as by 

 sensibility and voluntary motion, and the same necessity exists for 

 the re-establishment of its primitive composition. 



The transmutations necessary to vegetable life are occasioned by 

 irritants ; the aliment irritates the intestine, the blood irritates the 

 heart, &c. These movements are all independent of the will, and 

 generally (while in health) take place without the knowledge of the 

 individual ; in several parts, the nerves that produce them are even 

 differently arranged from those that are appropriated to sensation or 

 dependent on the will, and the very object of this difference appears 

 to be the securing of this independence. 



The nervous functions, that is, sensibility and muscular irritability, 

 are so much the stronger at every point, in proportion as their ex- 

 citing cause is abundant ; and as this cause or the nervous fluid is 

 produced by secretion, its abundance must be in proportion to the 

 quantity of medullary or secretory matter, and the amount of blood 

 received by the latter. 



In animals that have a circulating system, the blood is propelled 



