138 



ANIMAL. 



certain temperature, intercourse with the air 

 of the atmosphere, supplies of aliment, and 

 the access of light, and it is indubitable that 

 organized beings exhibit phenomena that 

 may be designated excitability, irritability, 

 vital force, &c., which are only other names 

 for these manifestations ; but it is also certain 

 that external conditions are of themselves ina- 

 dequate to originate manifestations of vitality, 

 and that the phenomena of living organized 

 beings, generally designated excitability, irrita- 

 bility, incitability, &c., are consequences of a 

 state of things to explain which they have been 

 conceived as causes, under the title of life, 

 vital principle, soul, &c. The term excitability 

 should be used in physiology, in the very 

 widest sense, to signify a property inherent in 

 organized matter generally, to be determined 

 to manifestations of activity under and in con- 

 formity with external influences (Tiedemann). 

 \Ve in fact see organized matter of every de- 

 scription the green matter of Priestley, con- 

 fervae, infusory animals, &c., acquiring organic 

 forms under the dominion of outward influences, 

 and every species of organized being existing 

 within a determinate circle of external agency. 



It were a grave mistake to suppose this 

 agency either chemical or mechanical in its na- 

 ture ; when of such potency as to act either 

 chemically or mechanically it is destructive 

 instead of productive of vital phenomena. 

 These phenomena, therefore, and external 

 influences are rather in opposition to one 

 another than identical. External influences ex- 

 cite organized beings to manifest their inherent 

 capacities; they do not bestow these capacities ; 

 all organized beings, indeed, and each parti- 

 cular tissue of every individual among them, 

 are excited in different modes by the various 

 influences from without ; the stimulus is iden- 

 tical, the effects are infinitely different. 



But organized beings, and especially ani- 

 mals, are not dependent on external influences 

 alone for the manifestation of their peculiar 

 properties; they have themselves the additional 

 power of engendering stimuli proper to arouse 

 into activity the various organs and systems of 

 which they are composed. The fluids circu- 

 lating through every part of their bodies may 

 be regarded in the light of the most generally 

 distributed stimuli of this description. The 

 nrevous system is another and important source 

 of excitation, the influence of which is felt in 

 every part of the organism of all animals above 

 the very lowest. The various instincts, appe- 

 tites, propensities, sentiments, and intellectual 

 faculties, also, which all emanate from the 

 nervous system, are inherent causes of a vast 

 variety of manifestations of activity among the 

 more perfect animals. There are yet other 

 stimuli of a mechanical, or chemical, or pecu- 

 culiar nature, which excite unusual or ano- 

 malous manifestations ; in this category may 

 be placed contagions of different kinds, the 

 causes of epidemic diseases, medicines, &c. 



With regard to the essence or cause of this 

 property of the organic globule to commence, 

 and of the perfectly developed organism to 

 manifest the various phenomena whose sum 



constitutes their vitality, and endows them 

 with their various cognizable properties, all we 

 can say is that it appears to inhere immedi- 

 ately in the particular state of matter which 

 composes them. What this state is in itself 

 we cannot tell ; but we are familiar with the 

 phenomena which ensue from, and which in- 

 deed reveal to us its existence. It is evidently 

 as diversified as species, and as the systems or 

 organs possessed by the individuals severally 

 composing these : there is a power the nisus 

 formations, the vis plastica, in the matter sus- 

 ceptible of formation, the organic globule, the 

 germ, which presides over and regulates its 

 acts; and there are powers inherent in the parts 

 or organisms to which the plastic force gives 

 rise, in accordance with which they manifest 

 the special acts that distinguish them. It 

 would be improper, however, to regard this 

 power or these powers as forces apart from 

 and other than the globules, germs or organisms 

 themselves ; in the present state of our know- 

 ledge we cannot separate exciting causes from 

 manifestations of activity; all we can venture 

 to say is that germs exist, that organisms exist 

 with inherent capacities of action in harmony 

 with the peculiar states of their constituent 

 elements, thus : the germ of the infusory ani- 

 mal exists with its inherent capacity to en- 

 gender an infusory animal, the germ of the 

 polype with its inherent power to produce a 

 polype; in the same way the various tissues, 

 vessels, glands, &c. of vegetables and animals 

 exist with their special capacities of excitation, 

 which are manifested in the particular functions 

 they severally perform. Excitability is there- 

 fore a multiform property and a consequence, 

 not a single peculiar power inherent in orga- 

 nized beings, the fundamental cause of their 

 actions and identical with or itself the living 

 principle. Dependent on the integrity and 

 continuance of the functions of nutrition, how 

 can it be the cause of these? Only manifested 

 in kind, with the occurrence of specific organs, 

 how can it be the cause of their several ma- 

 nifestations ? 



We are altogether in the dark with regard 

 to the mode in which the motions and other 

 actions of organized beings are performed, how 

 or by what law the globule that in the infusion 

 of organic matter is to become an infusory 

 animal moves, as well as of the manner in 

 which the contractility of a muscle is excited 

 by the stimuli fitted to call this quality into 

 action. The contractility of the infusoria, po- 

 lypi, medusae, and other similar tribes appears 

 to be peculiar. The motions exhibited by the 

 confervae, tremellae, and simplest vegetables 

 are also peculiar to them, they differ from 

 those manifested by the simplest animals in 

 being entirely under the influence of external 

 influences, and showing nothing like spon- 

 taneity. The tissues of all animals, even the 

 most complicated, show traces of a vital ten- 

 sion or contractility, different from simple 

 elasticity and not depending on muscularity; 

 the cellular membrane, skin, fibrous tissues 

 generally, excretory ducts, and vessels of all 

 descriptions tend to contract upon the parts 



