IRRITABILITY, AND MUSCULAR POWER. 107 



They are uniformly accompanied through their course by a number of 

 very minute nerves, which are chords or tubes that originate from the 

 brain, and branch out in every direction,either immediately from the brain 

 itself, or from some part of the spinal marrow, which is a continuation of 

 this organ ; by which means a perpetual communication is kept up between 

 the sensorium and the remotest part of the body, as we shall have further 

 occasion to notice hereafter.* Upon the 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 relaxa- 

 tion : ■ and it is chiefly by this curious contrivance that the animal system 

 is enabled to fulfil all its functions. The stimuH by which the fibres, 

 whether of motion or of sensation, are roused into action, are perhaps in- 

 numerable in the whole ; but a few general classes may easily be devised 

 to comprise all those by which they are ordinarily aflTected. And while, 

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

 fected 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 dif- 

 ferent offices intrusted to them, and no one interferes with that of ano- 

 ther. Thus the fibres of the external senses are affected by external ob- 

 jects ; 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 Hght, and remain uninfluenced by that of sound : and so 

 oi 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 stimuh, and the motions to which they give rise, 

 sensitive motions. 



Again, the very substances naturally introduced into many of the mus- 

 cular organs of the body, aud especially the hollow muscles, are sufficient 

 to excite 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 oflf 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 irri- 

 tation. 



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

 imprest upon by the different actions that are perpetually taking place all 

 over the system, through the medium of its own ramifications, or nerves, 

 that uniformly accompany 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 ex- 

 citing cause, and thus giving birth to a third class of stimuli, and of a very 

 extensive range, which are called stimuli of vohtion. While habit or as- 

 sociation becomes, in a variety of instances, a sufficient impulse to other 



* Ser. I, Lect* XV, 



