108 ON THE PRINCIPLE OF LIFE, 



application of any irritating or stimulating power, these fibres immediately 

 contract in their length, and upon the cessation of such power return to their 

 former state of relaxation : and it is chietly by this curious contrivance that 

 the animal system is enabled to fulfil all its functions. The stimuli by which 

 the fibres, whether of motion or of sensation, are roused into action, are per- 

 haps innumerable in the whole; but a few general classes may easily be de- 

 vised to comprise all those by which they are ordinarily affected. And while 

 by an admirable diversity of construction, some sets of fibres are only affected 

 by some sets of stimuli, other sets are only affected by others ; and in this 

 manner all the organs are compelled, as it were, to execute the different offices 

 intrusted to them, and no one interferes with that of another. Thus the 

 fibres of the external senses are affected by external objects ; they contract 

 and give notice of the presence and degree of power of such objects to the 

 brain, through the medium of the nerves, which, as I have just observed, 

 always accompany them, and which either terminate in or arise from that 

 organ : but while the irritative and sensitive fibres of the ear are excited only 

 by the stimulus of sound, and have no impression produced upon them by 

 that of light, those of the eye are excited only by the stimulus of light, and 

 remain uninfluenced by that of sound: and so of the other organs of external 

 sense. And hence we obtain a knowledge of one set or class of stimuli, 

 which from their acting upon the organs of sense, are called sensitive stimuli, 

 and the motions to which they give rise sensitive motions. 



Again, the very substances naturally introduced into many of the muscular 

 organs of the body, and especially the hollow muscles, are sufficient to ex- 

 cite them to a due performance of their functions : thus, the lungs are excited 

 to the act of respiration by the stimulus of the air we breathe, the stomach 

 to that of digestion by the stimulus of the food introduced into it; so the 

 heart and blood-vessels are excited by the stimulus of the blood ; and the 

 vessels that carry off the recremental materials by the different stimuli which 

 these materials contain in themselves. We hence obtain another class of 

 stimuli, which are denominated stimuli of simple irritation; and the motions 

 they produce, simple irritative motions, or motions of irritation. 



But the sensory, or brain, which thus receives notice generally, or is im- 

 pressed upon by the different actions that are perpetually taking place all over 

 the system, through the medium of its own ramifications, or nerves, that uni- 

 formly acccompany the irritable fibres, in many instances originates motions, 

 and thus proves a stimulus in itself. All voluntary motions are of this kind; 

 the will, which is a faculty of the sensorium, being the excitmg cause, and 

 thus giving birth to a third class of stimuli, and of a very extensive range, 

 which are called stimuli of volition. While habit or association becomes, in 

 a variety of instances, a sufficient impulse to other motions, and thus con- 

 stitutes a fourth class; which are hence named associate stimuli, or stimuli 

 of association. 



But though the muscular fibre is, perhaps, more irritable than any other 

 part of the system, the principle of irritability and a fibrous structure are by 

 no means necessarily connected ; for, while the cellular membrane is fibrous 

 but has no irritability whatever, the skin is not fibrous but is highly irritable. 



Hence solids and fluids are equally necessary to the perfection of the living 

 system. Food, air, and the ethereal gases, caloric, oxygen, and the medium 

 of electricity, are the stimuli by which it is chiefly excited to action ; and, by 

 their combination, contribute in some degree to the matter of the system 

 itself; but of the mysterious power that developes the organs and applies 

 the stimuli, that harmonizes the action and constitutes the life, we know 

 nothing. 



We see clearly, however, that the moving powers are, for the most part, 

 the muscles ; and it is a subject of perpetual astonishment to the physiologist 

 to observe the prodigious force which these vital cords are made capable of 

 exerting, and the infinite variety of purposes to which they thus become sub- 

 servient. And were it not that the whole universe swarms with proofs of 

 intelligence and design — were it not that there exists, to adopt the beautiful 

 words of the poet — 



