24 INTRODUCTION. 



the aliment irritates [or excites] the intestine, the blood irritates the heart, &c. These 

 movements are all independent of the will, and in general (while health endures) take 

 place without the cognizance of the me ; the nerves which produce them have even, 

 in several parts, a different distribution from that of the nerves affected by sensations 

 or subject to the will, and the object of the difference appears to be the securing of 

 this independence.* 



The nervous functions, that is to say, sensitiveness and muscular irritabiUty, are so 

 much the stronger at every point, in proportion as the exciting cause is more abundant ; 

 and as this agent, or the nervous fluid, is produced by secretion [or evolution], 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 circulation, 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 vigorously, and propel a greater quantity of 

 blood ; the nervous fluid becomes more abundant, and augments the local sensibility ; 

 this, in its turn, increases the irritability of the arteries, so that this mutual action may 

 be carried to a great extent. It is termed orgasm, and when it becomes painful and 

 permanent, inflammation. The irritation may also originate in the nerve, when it 

 experiences acute sensations. 



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

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



As each external sense is permeable only by particular kinds of sensation, so 

 each internal organ may be accessible only to such or such agent of irritation. Thus, 

 mercury irritates the salivary glands, cantharides excite the bladder, &c. I'hese 

 agents are what are termed specifics. 



The nervous system being homogeneous and continuous, local sensations and irrita- 

 tion debilitate the whole • and each function, carried too far, may enfeeble the others. 

 Excess of aliment thus weakens the faculty of thought ; while prolonged meditation 

 impairs the energy of digestion, &c. 



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

 were concentrated on a single point. 



A second irritation produced at another point may diminish, or divert as it is termed, 

 the first ; such is the effect of purgatives, blisters, &c. [denominated counter- irritation] . 



AH rapid as the foregoing enunciation is, it is sufficient to establish the possibility of 

 accounting for all the phenomena of physical life, by the simple admission of a fluid 

 such as we have defined, from the properties which it manifests.t 



* In the above sentence, there are distinctly mentioned the three I John Herschel, " (for which wonderfully constituted organ no other 

 sorts of nerves, the separate functions of which have been con- mode of action possessing the least probability has ever been devised), 

 ely demonstrated by Sir Charles Dell : viz., nerves of volMuii, \ be an electric pile, constantly in action, it may be conceived to dis- 



■which transmit the mandates of the will ; of sensation, which conv< 

 to the sensorium the impressions of the senses; ^ntX oi sympathy, 

 or involuntary movement, the reunion of the ramifications of which 

 in a plexus of knots, or ganglions, is intimated in the text, those of 

 the second class being distinguished by a swelling or ganglion near 

 their base.— Ed. 



t The unceasing chemical changes consequent upon vitality must 

 necessarily develope electricity ; and that the nernousjiuid is no other 

 than the electric, may be considered as proved by the identity of their 

 phenomena. Indeed, it has long been known that the transmission 

 of voltaic electricity along the nerves of a recently dead animal, 

 suffices to produce the most violent muscular action ; but the regula- 

 tion of that action, its e.iclusive direction to particular suites of 

 muscles, requires the vital impulse. " If the brain," remarks Sir 



charge itself at regular intervals, when the tension of the electricity 

 developed reaches a certain point, along the nerves which communi- 

 cate with the heart, and thus to excite the pulsations of that organ. 

 This idea is forcibly suggested by a view of that elegant apparatus, 

 the dry pile of Deluc, in which the successive accumulations of 

 electricity arc carried off by a suspended ball, which is kept, by the 

 discharges, in a state of regular pulsation for any length of time. We 

 have witnessed tlie action of such a pile, maintained in this way for 

 whole years, in the study of the above-named eminent philosopher. 

 The same idea of the cause of the pulsation of the heart appears to 

 have occurred to Dr. Arnott, and is mentioned in his useful and ex- 

 cellent work on Physics, to which, however, we are not indebted for 

 the suggestion, it having occurred to us independently many years 

 ago."— Discourse on the Study of Natural Philosophy, p. 343.— Ed. 



