INTRODUCTION. 15 



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 exciting 

 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 secreted matter, and the amount of blood received by the 

 latter. 



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

 through the arteries which convey it to its destined parts, by means of 

 their irritability and that of the heart. If these arteries be irritated, 

 they act more strongly, and propel a greater quantity of blood ; the 

 nervous fluid becomes more abundant and augments the local sen- 

 sibility ; this, in its turn, augments the irritability of the arteries, so 

 that this mutual action may sometimes be carried to a great extent. 

 It is called orgasm ; and when it becomes painful and permanent, 

 inflammation. The irritation may also originate in the nerve when 

 exposed to the influence of acute sensations. 



This mutual influence of the nerves and fibres, either intestinal or 

 arterial, is the real spring of vegetative life in animals. 



As each external sense is penetrable only by certain sensible sub- 

 stances, so each internal organ may be accessible only to this or that 

 agent of irritation. Thus, mercury irritates the salivary glands, 

 cantharides irritate the bladder, &c. These agents are called specifics. 



The nervous system being homogeneous and continuous, local sen- 

 sation and irritation debilitate the whole ; and each function, by 

 excessive action, may weaken the others. Excess of aliment weakens 

 the power of thought, while long-continued meditation impairs that of 

 digestion, &c. 



Excessive local irritation will enfeeble the whole body, as if all the 

 powers of life were concentrated in one single point. 



A second irritation produced at another part may diminish, or 

 divert, as it is termed, the first : such is the effect of blisters, 

 purgatives, &c. 



Brief as our sketch has been, it is sufficient to establish the possi- 

 bility of accounting for all the phenomena of physical life, from the 

 properties it presents, by the simple admission of a fluid such as we 

 have defined. 



